Risk of breast cancer was examined in relation to intake of dietary fiber and vitamins A, C, and E, and food groups which are sources of these dietary constituents, in a cohort of 56,837 women enrolled in the Canadian National Breast Screening Study. Between 1982 and 1987, 519 incident, histologically confirmed cases of breast cancer were identified among women who previously had completed self-administered dietary questionnaires. Their nutrient and food intake was compared with that of 1,182 women who had not developed breast cancer during the follow-up period. Women at the uppermost quintile level of dietary fiber intake had a 30 percent reduction in risk of breast cancer relative to that for women at the lowest quintile level (adjusted odds ratio = 0.68, 95 percent confidence interval = 0.46-1.00), and the reduction in risk persisted after adjustment (separately) for total vitamin A, beta-carotene, vitamin C, and alpha-tocopherol. Inverse associations of similar magnitude were observed in association with consumption of pasta, cereals (the trend for which was statistically significant), and vegetables rich in vitamins A and C. Smaller, statistically nonsignificant reductions in risk were observed with increasing intake of dietary retinol, beta-carotene, and vitamin C, but the magnitude of these associations was reduced after adjustment for other dietary factors. Vitamin E intake was not associated with altered risk of breast cancer.