Dietary fat and risk of breast cancer. The relationship between dietary fat and subsequent risk of breast cancer was studied in 3988 initially cancer-free Finnish women aged 20-69 y. During a follow-up period of 20 y, 54 breast-cancer cases were diagnosed. Risk of breast cancer was significantly inversely related to energy intake and nonsignificantly inversely related to absolute fat intake. A positive association between energy-adjusted total fat intake and occurrence of breast cancer was also observed. The relative risk in the highest tertile as compared with the lowest tertile was 1.7 (95% confidence limits 0.6-4.8). The corresponding relative risks were 1.4 (0.5-3.7) for saturated fatty acids, 2.7 (1.0-7.4) for monounsaturated fatty acids, 1.2 (0.6-2.8) for polyunsaturated fatty acids, and 2.2 (1.0-5.0) for cholesterol intake. Adjustment for different potential confounding factors did not alter the results. The present data suggest that breast cancer is associated inversely with energy intake and weakly positively with energy-adjusted fat intake. 