In an eight-year controlled clinical trial of a diet high in polyunsaturated vegetable oils and low in saturated fat and cholesterol in preventing complications of atherosclerosis, 846 men were assigned randomly to a conventional diet or to one similar in all respects except for a substitution of vegetable oils for saturated fat. Fatal atherosclerotic events were more common in the control group (70 v.48; P<0·05). However, total mortality was similar in the two groups: 178 controls v. 174 experimentals, demonstrating an excess of non-atherosclerotic deaths in the experimental group. This was accounted for by a greater incidence of fatal carcinomas in the experimental group. 31 of 174 deaths in the experimental group were due to cancer, as opposed to 17 of 178 deaths in the control group (P=0·06).