Exogenous hormone use as either oral contraceptives (OC) or hormone replacement therapy (HRT) was evaluated in reference to subsequent breast cancer risk in a cohort study of 20,341 Seventh-day Adventist women, residing in California, who completed a detailed lifestyle questionnaire in 1976 and who were followed for 6 years. During the follow-up period, 215 histologically confirmed primary breast cancers were detected in the cohort. The mean age at diagnosis was 66 years, indicating a primarily postmenopausal case series. In this cohort, after taking into account potentially confounding variables, current use of HRT (in 1976) was associated with a 69% increase in breast cancer risk, which was statistically significant (RR = 1.69; CI = 1.12-2.55). However, there was no strong increase in risk with increasing duration of use of HRT. Subgroups of women who did experience HRT associated increases in breast cancer risk included those women who had ever used HRT (RR = 1.39; CI = 1.00-1.94) and those with no history of maternal breast cancer (RR = 1.45), those women with prior benign breast disease (RR = 2.80), and those women who experienced menopause at 44 years of age or later (RR = 1.56). There was no substantial increase in breast cancer risk associated with use of OC in this population, although among women with exposure to both OC and HRT there was a suggested increase in risk (RR = 1.42; CI = 0.71-2.85).