That could mean that fat in the diet may have a small effect, Dr. Rossouw said, perhaps in some subgroups of women or over a longer period of time. He added that the study investigators would continue to follow the women to see if the effect became more pronounced.

　　While cancer researchers said they were disappointed by the results, heart disease researchers said they were not surprised that simply reducing total fat had no effect, because they had moved on from that hypothesis.

　　Of course, Dr. Libby acknowledged, the latest advice, to follow a Mediterranean diet and get regular exercise, has never been tested in a large randomized clinical trial. "If they did a study like that and it was negative," he said, "then I'd have to give up my cherished hypotheses for data."

　　The low-fat diet was not easy to follow, said Dr. Rowan T. Chlebowski, a medical oncologist at Harbor-U.C.L.A. Medical Center and one of the study's principal investigators. Women were told to aim for a diet that had just 20 percent of its calories as fat, and most fell short.

　　The diet they were told to follow "is different than the way most people eat," Dr. Chlebowski said. It meant, for example, no butter on bread, no cream cheese on bagels, no oil in salad dressings.

　　"If a physician told a patient to eat less fat, that will do nothing," he said. "If you send someone to a dietitian one time, that will do next to nothing." The women in the study had 18 sessions in small groups with a trained nutritionist in the first year and four sessions a year after that.

　　In the first year, the women on the low-fat diets reduced the percentage of fat in their diet to 24 percent of daily calories, and by the end of the study their diets had 29 percent of their calories as fat. In the first year, the women in the control group were eating 35 percent of their calories as fat, and by the end of the study their dietary fat content was 37 percent. The two groups consumed about the same number of calories.

　　Some medical specialists emphasized that the study did not mean people should abandon low-fat diets.

　　"What we are saying is that a modest reduction of fat and a substitution with fruits and vegetables did not do anything for heart disease and stroke or breast cancer or colorectal cancer," said Dr. Nanette K. Wenger, a cardiologist and professor of medicine at Emory University School of Medicine in Atlanta. "It doesn't say that this diet is not beneficial."

　　But Dr. Freedman, the Berkeley statistician, said the overall lesson was clear.

　　"We, in the scientific community, often give strong advice based on flimsy evidence," he said. "That's why we have to do experiments."