The authors report a case-control study of breast cancer based on an analysis of data collected by interview between the years 1957-1965 from women residing in the communities of Buffalo and Kenmore, New York. Prior reproductive factors, including a detailed lactation history, were examined for 453 white females with breast cancer and 1,365 white females without breast cancer who were selected randomly from the population of Buffalo and Kenmore. There is evidence of a negative association between length of nursing and breast cancer risk in premenopausal women which is not seen in the postmenopausal women. This apparent "protective" effect of lactation persists after statistical control for the potential confounding factors of age, parity, age at first pregnancy, age at menarche, and education. Cases are more likely than controls to have reported unsuccessful lactations due to "insufficient milk." The findings of this study, in conjunction with the authors' review of previously published studies that have examined prior lactation as related to breast cancer risk, suggest that there may be an independent negative association between nursing and subsequent breast cancer risk in premenopausal women. Whether this means that breast feeding is protective or that some women who are unsuccessful at lactation are at increased risk for subsequent breast cancer is not clear. A detailed reanalysis of existing data and more careful attention to detailed prior breast function/dysfunction in future studies are recommended.