This cross-sectional retrospective epidemiologic study investigated risk factors for low bone mineral density (BMD) in a group of 2297 women, 76% of whom were postmenopausal. Reproductive information, history of oral contraceptive use, BMD measurements, and other data were available from women presenting to 12 osteoporosis screening centers in 1986 and 1987. Each woman was classified into a BMD category based on the range of BMD measurements at her respective center. Menopause, increasing age and years since menopause, and decreasing body mass index were associated with low BMD. A history of oral contraceptive use was protective against low BMD (odds ratio = 0.35, 95% confidence interval = 0.23 to 0.53). Multivariate analyses confirmed this result and further demonstrated that increasing duration of use was protective. These data suggest that prior use of oral contraceptive agents is associated with higher levels of BMD and that the degree of protection from lower BMD is related to duration of exposure.