The side effects of a promising new experimental cancer therapy can be so severe that they may have caused the death of one of the advanced cancer patients in the study, the head of the National Cancer Institute research team says.

The death occurred about six weeks ago, but questions were raised yesterday about why it was not mentioned in an announcement last Wednesday by the NCI team. That announcement focused widespread national attention on the first experimental results of a new therapeutic approach with humans that uses natural substances to stimulate the body's immune defenses to fight cancer.

Dr. Steven A. Rosenberg, NCI's head of surgery, said yesterday that he had tried to follow scientific protocol by confining information to the first 25 patients described in an article published Dec. 5 in a medical journal. The death occurred in another group of patients, who were studied after the article was originally submitted for publication last summer, he said.

Dr. Arnold Relman, editor of the New England Journal of Medicine, which published the study, disagreed with the decision. He said it would have been in the "public interest" to disclose the death at the time the announcement was made last week.

Researchers have "an obligation not to allow expectations to grow to unrealistic proportions. A death due to therapy is an important, sobering fact that the public ought to be let in on even before it appears as a statistic in another paper," said Relman, who said he had not known of the death in publishing the preliminary findings.

The New England Journal generally follows a strict rule that authors should "refrain from promoting dissemination of data prior to publication and review in the journal," Relman said. Many researchers cite the rule in refusing to disclose findings in the news media for fear that their work will be rejected for subsequent publication.

Relman said, however, that "our rule allows scientists to talk about their results when it is clearly in the public interest."

Rosenberg disclosed the death in an interview Sunday on CBS News' "Face the Nation," in which he emphasized the serious toxic side effects that can accompany the new therapy. Noting that many patients experienced serious weight gain because of fluid retention, he said, "We have, in fact, since the 25 patients reported in the article in the New England Journal of Medicine, even seen one death due to the treatment itself."

Rosenberg noted in an interview yesterday that he had initially confined his "comments to information in the medical literature . . . . There was certainly no attempt to conceal any information."

He and Relman emphasized that a single death is not unexpected in attempting a new cancer therapy on advanced patients who are so ill that they cannot be helped by any conventional treatment. The death occurred in a patient with a severe form of skin cancer known as melanoma that had subsequently spread to several organs.

NCI's news release last week noted side effects in the first 25 patients, but said they disappeared after the therapy was discontinued.

Paul Van Nevel, associate director for cancer communications, said yesterday that his office had not been aware last week of the death. "We concentrated solely on the first 25 patients," he said. "If we had known about it, we certainly would have advised that it be included."