A case-control study of breast cancer among Black American women was conducted in seven hospitals in New York City from 1969 to 1975. Results are reported for 127 cases and 317 controls. Compared to women with a first birth before age 19, those with a first birth after 25 had a relative incidence rate for breast cancer of 3.8 and 2.2 for the pre- and postmenopausal age-groups, respectively. Compared to nulliparous women, parous women had a relative incidence rate of 0.6 for premenopausal and 0.7 for postmenopausal women. The incidence rate of breast cancer for women with a menopause after age 49 was estimated to be 3.1 times that of women with a menopause before age 45. Thus, the known risk factors for breast cancer among Whites are also related to the etiology of the disease among Blacks. The incidence rate of breast cancer has increased among younger Blacks since 1947 and is now similar to that among younger Whites. However, among older women, the incidence rate is still appreciably higher for Whites. The most likely explanation of this pattern is that Black women born since about 1925 are being exposed at the same frequency as White women to the causes of breast cancer.