To further explore whether breast cancer risk factors are the same for black women and white women, the authors investigated several biologic, cultural, and social factors in a 1980–1982 case-control study of non-Hispanic black subjects (490 cases, 485 controls) and non-Hispanic white subjects (3, 934 cases, 3, 901 controls) aged 20–54 years. Logistic regression analyses indicated that blacks and whites shared four risk factors at a comparable magnitude (age at first full-term birth, parity, surgical menopause, and history of benign breast disease). For two observed risk factors, the magnitude (breast feeding) and pattern (family history of breast cancer) of the relation were different in blacks and whites. The relative risks of breast cancer among black women who had first-degree relatives (odds ratio (OR) = 1.61) and second-degree relatives (OR = 1.71) with breast cancer were similar, whereas the relative risk among white women who had first-degree relatives (OR = 2.16) was distinctly larger than for those who had second-degree relatives (OR = 1.44) with breast cancer. The relation of early age at menarche appeared negligible for blacks although significant for whites aged 12 and under (OR = 1.26). The results also indicated that natural menopause, oral contraceptive use, and cigarette smoking may have different, and more complex, relations to breast cancer among blacks and whites. Am J Epidemiol 1992; 136: 1445–56