This study examines the associations between endogenous sex steroids and bone mineral density (BMD), using data from a geographically defined cohort in Rancho Bernardo, California. Participants were community-dwelling women and men aged 50-89 years who took part in a study of endogenous sex steroid measurement between 1984-1987 and who had BMD measured in 1988-1991. Those taking corticosteroids or estrogen at the time of sex steroid determination were excluded. The main study outcomes were BMD of the ultradistal radius, midshaft radius, lumbar spine, and total hip by sex steroid level, adjusted for age, body mass index, cigarette smoking, alcohol consumption, leisure exercise, use of thiazides, thyroid hormones, and former estrogen use (women only). At the time of the hormone measurements, the mean age of the 457 women was 72.1 years and that of the 534 men was 68.6 years. A statistically significant positive relation was seen between bioavailable estradiol and BMD at all sites in women and men. Total estradiol was significantly associated with BMD at all sites in women and at all but the ultradistal radius in men. Estrone had a global effect on BMD in women and was not measured in men. Higher bioavailable (but not total) testosterone levels were associated with higher BMD of the ultradistal radius, spine, and hip in men and the ultradistal radius in women. Dehydroepiandrosterone was positively associated with BMD of the midradius, spine, and hip in women and was not associated with BMD at any site in men. Of the sex steroids tested, bioavailable estrogen was most strongly associated with BMD in both women and men. We conclude that endogenous sex steroid levels are significantly related to bone density in older women and men. Individual variation in age-related bone loss may be partially accounted for by alterations in sex steroid levels with aging. Further study to elucidate safe environmental and medical methods to maintain optimal sex steroid levels in old age is needed.