The swift boats, like the one John Kerry captained, were sitting ducks. Hiding in the dense jungle along the riverbanks of the Mekong Delta, the Viet Cong could open up on the Americans with machine guns, mortars and rockets--and vanish before the Americans could effectively shoot back. So the U.S. military adjusted, dropping tons of herbicide on the foliage to strip the enemy of its cover. "They just told us they sprayed something to kill the bushes," recalls Mike Medeiros, who served with Kerry aboard PCF (Patrol Craft Fast) 94 in the winter of 1969. "It looked like a moonscape... You saw skeletal remains of trees everywhere. It was like, whatever they're using is some serious stuff."

Some of Kerry's men, contacted last week by NEWSWEEK, don't recall being directly sprayed, or saw planes or choppers dropping the herbicides in the distance. Other Swift Boat crews patrolling near Kerry's boat say they were doused. But everyone, including Kerry, realizes today that they fought in a highly toxic environment. "We know they used defoliants, at least I knew they used defoliants, because it was all around us," says Kerry. "We'd often see the C-130s flying over us--they'd come down along the river and drop this stuff on us," recalls Wade Sanders, who served with Kerry in the delta, on different boats but sometimes on the same patrols along the Cambodian border. "The wind would carry the mist right onto us. But they didn't tell us what it was. We thought it was mosquito spray." The "stuff" was Agent Orange, which, over time, can be lethal to humans as well as to most other living things. "We used to bathe in those rivers and swim in the water to cool off," says Sanders. "Agent Orange was everywhere, all around us, but no one ever told us not to go in the water." Over the years, says Sanders, he and Kerry "have had many discussions about our exposure to Agent Orange."

In his first public comments about his experience with Agent Orange, Kerry told NEWSWEEK, "I don't think I saw as much as he [Sanders] did, but I saw some of it, and I know we were in the water... That was just the nature of life on a boat down there. It was a reality." But, Kerry said, "I've never really thought about it... I don't think about it in a personal sense."

Agent Orange was one of the many tragedies of Vietnam. It may have killed or sickened, via long-incubating cancers and nerve disorders, thousands of American soldiers and sailors (not to mention many more Vietnamese). The government, which once avidly drenched the delta with the poison, has awakened to its dangers. "The environmental hazard of the battlefield--that could be equally as deadly as a bullet wound," said Anthony J. Principi, secretary of Veterans Affairs last year. "We learned that the hard way after Vietnam with Agent Orange." Adm. Elmo Zumwalt, the charismatic Navy commander who ordered the spraying of the Mekong Delta, became a crusader for vets suffering from diseases related to Agent Orange. When Zumwalt's son--like John Kerry, a Swift Boat captain--died of lymphoma in 1988 at the age of 42, Adm. Zumwalt came to blame his son's death on the Agent Orange campaign he had commanded. Kerry, who always preferred action over personal revelation, has dealt with the tragedy his own way. Kerry's health could become a political football (though it's worth remembering that Dick Cheney's heart condition raises questions about the Republican ticket). Ever the stoic, often a loner, Kerry is likely to tough it out. In his interview with NEWSWEEK, his tone was calm, unperturbed.

As a U.S. senator, Kerry has fought for years to help fellow veterans suffering from cancer, nerve and skin disorders and other diseases linked by medical science to chemical dioxins contained in Agent Orange. He was a chief sponsor of the 1991 legislation that now affords some 10,000 exposed veterans up to $2,300 a month for various disabilities. Kerry recently tried unsuccessfully to help Tommy (Trees) Forrest, who served on different boats but in the same combat area, to win benefits for cancers growing on his liver. "I personally observed the spraying of Agent Orange," Kerry wrote in a letter to Veterans Affairs on June 25, 2003. "I hereby testify with absolute certainty that Thomas G. Forrest and anyone else in that area of operations were definitely exposed to Agent Orange." On the same day, he wrote the same testimonial for one of his closest friends, Giles Whitcomb, a Naval Intelligence officer who served with Kerry in the delta and died last year of lymphoma.

Kerry himself was successfully operated on last year to have his cancerous prostate removed. Did Agent Orange put him at risk? Is he still at risk because of his exposure to the toxic chemical? "I presumed and I still presume, it is my belief that my particular cancer was hereditary and part of my genes," Kerry told NEWSWEEK. "It's gone now. I'm in perfect health, and I'm not concerned about something." Kerry's father died of prostate cancer at the age of 85, and doctors often look first at hereditary factors to predict a patient's risk. About one in six American men gets prostate cancer; roughly a quarter of them between the ages of 55 and 64 (Kerry is 60). The survival rate for those, like Kerry, whose cancer is caught early and undergo surgery is very high.

Still, prostate cancer is one of roughly a dozen diseases that the VA now considers "presumptive" evidence of exposure to Agent Orange. In 1996, the VA began compensating any vet diagnosed with prostate cancer on the ground that it may have been caused by Agent Orange. According to the VA, Kerry himself is entitled to receive Agent Orange benefits, but the wealthy Kerry has not asked for them.

It is impossible to judge the health risks to Kerry or any other veteran, says Dr. David Tollerud, who has overseen several government-sponsored studies on the health effects of Agent Orange, without knowing how often they were exposed, and how directly. A team of researchers at Columbia University has developed a computer model that can be used with detailed military records to assess precise exposure levels. To prepare such an assessment for Kerry, "we'd need the daily log of his Swift Boat," says Jeanne Stellman, a Columbia public-health professor who is overseeing the study. It is significant, she says, that Kerry was in the Mekong Delta, "the most heavily sprayed area in Vietnam, particularly in 1968 and 1969." On the other hand, Spellman cautions, from a purely statistical viewpoint, Kerry stands a greater risk of getting assassinated than getting cancer as a result of his exposure to Agent Orange.

The evidence on the level of Kerry's exposure is unclear. Jim Wasser, Kerry's second in command aboard PCF-44, recalls seeing Agent Orange clouds in the distance, not raining down directly. "But air currents took it a long ways," he says. Medeiros, who served with Kerry on a different boat, PCF-94, doesn't recall seeing any spray. (Both Wasser and Medeiros are healthy; "I look at every day as a bonus," says Wasser.) Forrest, who was on boats patrolling the same rivers as Kerry (including PCF-98, the same boat as Wade Sanders), recalls, "They'd fly overhead and release it in a mist. The bad thing about it was we got fish off the Vietnamese. We'd pick up wood and barbecue and that stuff was coated with Agent Orange. We'd bathe in the water, and Agent Orange dioxins aren't soluble." Kerry recalled to NEWSWEEK, "I'm not sure that I remember a specific incident of being doused, but I do remember that's what they were doing."

Some who have spent time with Kerry believe that he is haunted by the subject of Agent Orange. Historian Douglas Brinkley remembers getting a phone call from Kerry last December after the senator had just finished reading the galleys of Brinkley's new book, "Tour of Duty: John Kerry and the Vietnam War." One of Kerry's only criticisms, recalls Brinkley, was that "I didn't write enough about Agent Orange... He was obsessed with Agent Orange." Brinkley speculates that Kerry has been affected by the tales of woe from delta vets who are just starting to feel the sometimes delayed impact of Agent Orange. He was particularly moved by the recent death of his buddy Giles Whitcomb, a Harvard grad whom Kerry befriended in training and stayed close to all the way through the war (they even double-dated). "The devastating effects of Agent Orange are just coming to fruition. When John talks about these veterans, I think it's visceral," says Brinkley. "There's something going on there, and it may underneath it all be personal--like, 'I may have been doused, too'."

Kerry denies that he broods at all over Agent Orange. He has not released his medical records, but he promised to release a summary: "I'd be happy to have my doctor release something... I just had a checkup about three weeks ago. My blood is perfect. My EKG was perfect." Kerry's doctor, Patrick Walsh, chief of urology at Johns Hopkins, told NEWSWEEK that "there have been no signs of any recurrence of cancer cells in any of Kerry's follow-up tests," and that he "firmly believes that Kerry's history of prostate cancer is behind him." Kerry says, "My doctor says that I'm at lower risk than most people my age for everything you can think of because I have the heart of a marathon runner and the blood pressure of a marathon runner and I'm in great shape." As for worries that Agent Orange will make him sick, "I do not have that concern. I do not live with it. I feel very confident and it's just a feeling I have. I can't explain it. But that's how I feel."