Early childhood nutrition may affect the subsequent risk of breast cancer in adulthood. We examined the association of having been breastfed with risk of breast cancer in a case-control study of women age 40-85 years in western New York. Cases (N = 528) had newly diagnosed primary, pathologically confirmed breast cancer; controls (N = 602) were randomly selected from the same community and were frequency matched on age. Having been breastfed was associated with decreased risk. The multivariate adjusted odds ratio was 0.74, and the 95% confidence interval was 0.56-0.99. We found little difference in the association for pre- and postmenopausal women despite a much higher frequency of breastfeeding among the older women. These findings indicate that early nutriture in general and bottle feeding in particular may relate to breast cancer development in adulthood.