Two prominent prostate cancer experts have been threatened for opposing approval of a controversial new drug and are being protected by bodyguards as they attend the nation's largest cancer conference here.

The experts, Dr. Howard Scher, of Memorial Sloan-Kettering Cancer Center, and Dr. Maha Hussain, of the University of Michigan, have received e-mail and other threats, said spokeswomen for Sloan-Kettering and for the cancer conference.

It was not known who sent the threats. But it was clear that the doctors' public stance against the drug, Provenge, has inflamed the passions of some men with prostate cancer and patient advocates, who say the drug would offer hope to desperate patients with few or no alternatives.

Many investors also have made big bets on the stock of the drug's developer, Dendreon, a Seattle biotechnology company, whose stock has fluctuated wildly based on prospects for the drug. Some investors stood to profit if the drug was approved and others if it was rejected.

The FDA said in May that it would not approve Provenge without more evidence that it was safe and effective.

Patients with incurable diseases often advocate for the approval of new drugs even if the data supporting them are not perfect. But threats to those who take an opposing position on a drug take such advocacy to a new level.

That could discourage rational discussion of drugs or deter experts from serving on advisory panels to the Food and Drug Administration, where Scher and Hussain first voiced their opinions.

"Intimidation or harassment is going to make qualified people think twice about serving in national positions," Hussain said in a brief conversation here at the annual meeting of the American Society of Clinical Oncology. She said she would continue for now to serve on FDA advisory panels.

Scher also was reluctant to comment, saying, "This is a situation I've never been in before." But he seemed distressed that other doctors were not publicly rallying to his support. "There's no one else standing up and saying this is ridiculous," he said.

When he gave a talk at the conference on Sunday, Scher was accompanied by three men wearing suits and earphones but without mandatory conference name badges.

A spokeswoman for the oncology association said that it had been working with the two doctors and was increasing security at the conference.

Christine Hickey, a spokeswoman for Sloan-Kettering, said that Scher had received e-mail messages and telephone calls, including one e-mail titled "your murder." A copy of his biographical page on the Sloan-Kettering Web site was defaced.

Provenge has become the latest focal point of a long-running and sometimes bitter debate about the degree of evidence needed for approval of drugs for life-threatening diseases. An FDA advisory panel endorsed the effectiveness of the drug by a 13-to-4 vote in March.

Provenge had extended lives in two small clinical trials, although the results were somewhat weak by various statistical standards. The panel voted that the drug was generally safe but that there were signs it could increase the risk of strokes.

But both Scher and Hussain, who, unlike most of the panel members, actually treat patients who have prostate cancer, argued that the evidence fell short of proving the drug worked and that they did not want to give patients false hopes.