Dietary fat in midlife has not been associated with breast cancer risk in most studies, but few have followed women beyond one decade. The authors examined the relation of dietary fat, assessed by repeated questionnaires, to incidence of postmenopausal breast cancer in a cohort of 80,375 US women (3,537 new cases) prospectively followed for 20 years between 1980 and 2000. The multivariable relative risk for an increment of 5% of energy from total dietary fat intake was 0.98 (95% confidence interval: 0.95, 1.00). Additionally, specific types of fat were not associated with an increased risk of breast cancer. Furthermore, secondary analyses indicated no differences in breast cancer risk by estrogen receptor or progesterone receptor status. However, stratification by waist circumference indicated a significant decrease in breast cancer risk for participants with a waist circumference of 35 inches (88.9 cm) or greater (p-trend = 0.04). None of the latency intervals investigated were associated with an increased risk of breast cancer. In addition, fat intake before menopause was not related to risk of postmenopausal breast cancer. These results suggest a reduction in breast cancer risk for women with insulin resistance syndrome who consume high-fat diets and no association between specific sources of fat during midlife and risk of postmenopausal breast cancer.