When Jason Giambi showed up at the New York Yankees' spring-training camp in Tampa this year, he looked as if he were auditioning for a Hollywood sequel: "Honey, I Shrunk Myself." His neck seemed to have dropped several sizes, his shoulders had new slope, and his torso could no longer be described as hulking. Only months earlier Giambi had been named as a client of BALCO, a Bay Area lab accused of supplying illegal performance-enhancing drugs to elite athletes. So Giambi's physical transformation inevitably led to speculation that the slugging first baseman had stopped using steroids. Giambi publicly denied ever taking such drugs.

Last week the Yankee's denials were exposed as lies: in federal grand-jury testimony taken in 2003 and leaked to the San Francisco Chronicle, Giambi not only confessed that he had used steroids over several seasons, but that he had injected himself with both synthetic testosterone and human growth hormone. Giambi was one of a succession of prominent athletes paraded before the grand jury a year ago. Now some of America's most illustrious sports heroes--including Barry Bonds and Marion Jones--are once again center stage in what has become the biggest drug scandal in U.S. sports history.

Just one day after the Giambi revelations--including his claim that Bonds's personal trainer, Greg Anderson, provided him with banned drugs--it was Bonds's turn to see his grand-jury testimony leaked to the Chronicle. The newspaper reported that Bonds admitted using two BALCO products: a clear substance taken orally and a cream rub, both of which, investigators say, contained steroids. But Bonds testified that he believed he was using a nutritional supplement and a balm for arthritis.

Four men, including Anderson and BALCO founder Victor Conte, have been indicted for distributing illegal drugs. But Bonds appears to be the prime target. "This has always been the Barry Bonds show," his lawyer, Michael Rains, said Friday. He contends that having come up with a "goose egg," the Feds are using leaks to smear Bonds. Asked if Bonds's unwitting use of steroids was a case of willful ignorance, Rains replied, "It was more blind faith in his best friend."

However, after BALCO boss Conte's explosive interview with ABC News's "20/20," some suspect he's behind the new leaks. In the program, aired last Friday night, Conte said he began supplying illegal drugs to sprinter Marion Jones shortly before the 2000 Sydney Olympics and taught Jones to inject herself with human growth hormone. Last year the U.S. Anti-Doping Agency conducted an inconclusive investigation of Jones, who won five medals in Sydney. It was unclear why Conte singled out Jones while dodging questions about other clients such as Bonds. Jones said Conte's claims were "not true... the truth will come out," and her attorney, Richard Nichols, denounced the tale as "unsubstantiated allegations" by "a man facing a 42-count federal indictment."

Neither Giambi nor his attorneys made public comments last week. But even amid all the rumors and speculation, Giambi's confession was shocking for its graphic description of--and casual attitude toward--his rampant use of banned drugs. Giambi testified under a grant of immunity, as did all the athletes appearing before the grand jury. According to the Chronicle account, he admitted that he had already been injecting a steroid when he first met Anderson, after the 2002 season. He testified that the trainer introduced him to two steroid compounds: "the clear," taken as drops on the tongue, and "the cream," which is rubbed on the body. Anderson also started Giambi on a regimen of testosterone and human-growth-hormone injections. Giambi said he injected HGH under the skin in the folds of his stomach. Asked by the prosecutor if the testosterone was injected in the same fashion, Giambi said no, "you'd put it in your a--." But Giambi said Anderson never indicated that Bonds was also using illegal drugs. "You know, I assume because he's Barry's trainer... but he never said one time, 'This is what Barry's taking, this is what Barry's doing.' He never gave up another name."

Giambi's name alone should be enough to send major-league baseball reeling. Even coming off one of its most thrilling seasons, with record attendance, the sport will not easily weather a scandal of this magnitude. There are many who believe baseball ignored ample evidence of drug abuse while the assault on home-run records was helping to recapture fans after the 1994 strike. At the very least, there is little argument that MLB (along with the powerful Players union) produced a shamefully weak drug-testing program. While Giambi confessed to repeated violations of federal law, he is unlikely to have broken any MLB rules. Baseball didn't start testing until the 2003 season, and only this year did violations carry any, even minimal, sanctions. On Friday, Sen. John McCain said he would introduce legislation in January mandating minimum drug-testing standards for pro athletes if baseball doesn't enact a new policy cracking down on cheats.

This scandal goes beyond tarnished records and reputations. In 2002 Ken Caminiti, the National League MVP in 1996, became the first retired player to admit he had taken steroids--and he estimated that half the players were using them. So Caminiti wouldn't be shocked by the past week's revelations, had he lived to see them. But Caminiti died from a drug overdose during this year's playoffs, the end of a downward spiral that, he said, began with steroids. Giambi was sidelined for half the 2004 season with mysterious ailments. The New York Daily News reported that Giambi had a tumor on the pituitary gland. Giambi has indicated that his condition is completely unrelated to drugs. But unless he told that to a grand jury, there's no reason to believe it's true. And baseball could find itself moving from a historic celebration toward the most tragic chapter in its history.