A prostate cancer drug developed by the Seattle biotechnology company Dendreon prolonged the lives of men in a decisive clinical trial, the company announced Tuesday morning. The widely anticipated results could pave the way for the drug, called Provenge, to become the first so-called therapeutic cancer "vaccine" to win approval in the United States after numerous failures of such drugs.

"This looks like a proof of concept that cancer vaccines can and do work," said Jeffrey Schlom, an expert on the vaccines at the National Cancer Institute. Such vaccines work by harnessing the patient's own immune system to fight the cancer. But the success in the trial could revive complaints about the Food and Drug Administration, which two years ago declined to approve Provenge despite an endorsement by one of the agency's advisory committees.

The F.D.A.'s decision two years ago ignited an outcry from some prostate cancer patients and from investors in Dendreon, who said the agency was being unreasonable and denying patients a treatment that might work. Tensions ran so high at one point that two prostate cancer specialists, who had urged the F.D.A. not to approve the drug, attended a major conference accompanied by bodyguards, saying they had been threatened.

"Since that delay we have lost a lot of good men," Ted Girgus of Bellingham, Wash., who has advanced prostate cancer, said Tuesday, calling the F.D.A. decision "a punch in the stomach." Mr. Girgus, 65, who also owns Dendreon stock, said patients like himself are "looking into the abyss." "We know what our prognosis is," he said. "But we want a chance."

Dendreon did not reveal the actual results of its trial, saying they would be presented at a urology meeting on April 28. But Mitchell Gold, the company's chief executive, told analysts on a conference call, "It was an unambiguous hit on the primary endpoint of overall survival." He said the outcome met the goals the company and the F.D.A. had agreed upon and that the results were consistent with those seen in earlier trials of Provenge. In an interview Dr. Gold said Provenge would have had to reduce the risk of death by 22 percent compared to a placebo to meet the F.D.A. requirements for statistical significance.

The treatment requires three infusions spaced two weeks apart, rather than periodic infusions over the course of months, as is common with chemotherapy. Provenge's side effects also appear to be milder than those of Taxotere, some doctors say. "Compared to standard chemotherapy, it's just a whole lot easier for patients," said Dr. David Penson, associate professor of urology at the University of Southern California. He was an investigator in the trial and was a consultant to Dendreon a few years ago.