"The president has cancer" is always an arresting phrase. But by the time we got confirmation of Bill Clinton's case last week, he had already been cured. Doctors scraped a suspicious lesion from the president's back during his annual physical Jan. 12, and tests later revealed it was a basal-cell carcinoma. Though it qualifies as a malignancy, BCC is nearly as common as gray hair--and far more treatable. Recognized early, it leaves nothing but a small scar to remind you of the sun's more serious hazards. Unfortunately, BCC is on the rise in this country, and so are its deadlier cousins.

Skin cancer typically affects people over 50, but the pattern is starting to change. "We used to see [BCCs] mainly in old people," says Dr. Martin Weinstock, the Brown University dermatologist who heads the American Cancer Society's skin-cancer advisory group. "Now we're seeing them in people in their 20s and 30s." Sun exposure is a key risk factor for all three of the leading skin cancers--BCC, squamous-cell carcinoma and malignant melanoma--and incidence of all three has risen in recent decades as we've shed our clothes and spent more leisure time outdoors. Americans now develop nearly a million BCCs annually, and malignant melanomas have more than doubled, growing from fewer than six new cases per 100,000 people in 1973 to about 14 cases per 100,000 last year.

Basal-cell carcinomas are the most common of all skin cancers. They show up mainly in fair-skinned Caucasians, as pearly white scars or rough red scabs. And because sun exposure is the primary trigger, the risk is highest among people who live in pleasant climates. "You take people whose genes evolved for northerly climates and put them in places like California, Arizona, Texas and Florida, and of course you will see an increase," says Dr. Paul Weber, a dermatologist in Ft. Lauderdale. "The farther these people move from their genetic home base, the greater the chance that they will experience skin cancer."

Fortunately BCCs, which account for 80 percent of all skin cancers, rarely cause serious harm. Basal cells form the bottom layer of the epidermis, or outermost layer of the skin, and they don't survive well outside their native environment. As a result, the simple scrape-and-burn procedure that Clinton underwent during his physical is almost always curative. Left untreated, BCCs may slowly invade neighboring tissues. "They can destroy a nose, an ear, an eye," says Dr. Alan Moshell of the National Institute of Arthritis and Musculoskeletal and Skin Diseases. "They can erode the skull or eat through a vital artery, causing the person to bleed to death." But unlike other malignancies, BCCs rarely seed new tumors elsewhere in the body. As a result, they cause only a few hundred deaths each year. No one even keeps an official count.

If only the other skin cancers were so courteous. Squamous-cell carcinoma, the second most common variety, looks and acts a lot like BCC but is slightly more prone to metastasis. Malignant melanoma, the cancer that John McCain and Maureen Reagan are now battling, is a whole different story. It starts among the pigment- producing cells of the epidermis and spreads readily to other parts of the body. And once it takes root in a kidney, lung or liver, it can be dif-ficult to treat. Chemotherapy has little effect, and though therapeutic vaccines and other new treatments have shown great promise, they often fail. The result is that while melanoma makes up just 4 percent of all skin cancer, it accounts for almost 80 percent of the deaths. Last year alone it claimed an estimated 7,700 lives in this country.

Anyone can develop melanoma, but several factors raise a person's risk. In addition to fair skin and extensive sun exposure (especially during childhood and adolescence), the key risk factors include having moles--which can spawn melanomas without much outside encouragement--and a family history of the condition. Experts urge people who meet any of these criteria to inspect their bodies regularly, leaving no region unchecked. Any strangely colored spot that has ragged borders, an asymmetrical shape and the diameter of a pencil eraser should be seen by a doctor. So should a mole whose appearance changes.

Clinton is now cancer-free, but there is a catch. People who develop one BCC are at increased risk for future skin cancers-- including melanomas. The president will now get dermatological checkups in addition to regular physicals, and he'll have an extra incentive to follow the "slip-slop-slap" program for staying healthy (slip on a shirt, slop on some sunscreen, slap on a hat). We would all be wise to join him.