A cohort study of fat intake and risk of breast cancer. Between 1982 and 1987, 519 newly incident, histologically confirmed cases of breast cancer were identified in a cohort of 56,837 women enrolled in the Canadian National Breast Screening Study. These women had completed a dietary questionnaire before the occurrence of their breast cancer, and this has been used to estimate their intake of dietary fat and several other nutrients. There is evidence of a positive association between breast cancer and total fat intake, with a relative risk of 1.35 (95% confidence interval, 1.00-1.82) per 77 g per day, and some evidence of a dose-response relationship (P = .052). 