This case-control study included 183 Japanese and 161 Caucasian patients with breast cancer and equal numbers of matched hospital and neighborhood controls. The following factors were found to be associated with increased breast cancer risk in both groups of women: family history of breast cancer, history of benign breast disease, early age at menarche, late age at first childbirth, and late menopause. We noted a weak suggestion that the patients ate more saturated fats and oleic acid and took replacement estrogens for a longer period than did the neighborhood, but not hospital, controls. However, none of these differences was statistically significant.