Background: The relation between breastfeeding and breast-cancer risk has been examined in many studies; some have reported no association, and others a reduced risk, particularly among premenopausal women. In the only prospective cohort study, no association was found. We have assessed prospectively the association between breastfeeding and incidence of breast cancer among 89 887 women in the US Nurses' Health Study. Methods: In 1986, participants were asked about the number of months they breastfed for all their children combined. Parous women with no history of cancer were included in this analysis. During 6 years of follow-up (513 015 person-years), 1459 invasive breast cancer cases were diagnosed. Findings: Relative to women who had never breastfed, no significant overall association was found - after adjusting for established risk factors for breast cancer - between a history of having breastfed and subsequent development of breast cancer (relative risk [RR] 0.93, 95% Cl 0.83-1.03). No inverse trend was observed with duration of breastfeeding; women who breastfed for 2 years or longer had a RR of 1.11 (0.90-1.38). Among women who had given birth only once, women who had breastfed their child experienced a lower incidence of breast cancer (R·R 0.68, 0.46-1.00). Among premenopausal women, who tended to be near menopause due to the age structure of the cohort, the RR of breast cancer for those who had lactated was 1.16 (0.89-1.50). Premenopausal women who had lactated for 1 year or more had a RR of 1.10 (0.78-1.57). Interpretation: These data suggest that there is no important overall association between breast-feeding and the occurrence of breast cancer.