The association between dietary antioxidant vitamin intake and the risk of breast cancer was examined in a prospective study of 34,387 postmenopausal women in Iowa. Intakes of vitamins A, C, and E and of retinol and carotenoids were assessed in 1986 by mailed semiquantitative food frequency questionnaire. Through December 31, 1992, 879 incident breast cancer cases occurred in this cohort. There was little suggestion that breast cancer risk was associated with differences in intake of these vitamins. For example, from the lowest to highest total vitamin A intake categorized by quintiles, the age-adjusted relative risks of breast cancer were 1.0, 0.95, 1.17, 1.20, and 0.90 (p trend = 0.92). Similarly unremarkable relative risk patterns were seen for the intakes of vitamins C and E and of retinol and carotenoids. These findings were not altered after adjustment for breast cancer risk factors or in analyses confined to women who reported no supplemental vitamin intake. Exclusion of cases that occurred in the first 2 years of follow-up, under the assumption that women may have increased intake of these vitamins in response to preclinical symptoms of breast cancer, did not suggest an inverse association of these vitamins with the risk of breast cancer. Women who reported consuming at least 500 mg/day of supplemental vitamin C had a relative risk of breast cancer of 0.79 compared with women who did not take supplemental vitamin C, and women who reported consuming more than 10,000 IU/day of vitamin A had a corresponding relative risk of 0.73. However, these relative risks were not statistically significant. These results provide little evidence that intake of these vitamins is associated with breast cancer risk.