Estrogen is a critical hormone for bone homeostasis in men, but no information is available on the role of estrogen metabolism among men. The aim of this study was to evaluate the effect of estrogen hydroxylation on male bone mineral density (BMD). Participants consisted of 61 healthy Caucasian males (mean age 66.6 +/- 1.0 years). Urinary estrogen metabolites were measured by enzyme-linked immunosorbent assay, serum estradiol by ultrasensitive radioimmunoassay, sex hormone binding globulin by radioimmunoassay, and BMD of the lumbar spine and the proximal femur by dual-energy X-ray absorptiometry. Active estrogen metabolites, 16alpha-hydroxyestrone (16alphaOHE(1)) and estriol (E(3)), positively correlated with adjusted BMD in all regions of the proximal femur (all P < 0.05) but not at the lumbar spine, and those in the highest tertile of urinary 16alphaOHE(1 )had the highest BMD. Free estradiol index (FEI) also positively correlated with BMD of the total hip, femoral neck, and intertrochanter (all P < 0.05), while there was no correlation between BMD with inactive metabolites (2-hydroxyestrone and 2-methoxyestrone) and serum testosterone. Multiple regression analysis showed 16alphaOHE(1), FEI, and body mass index are important independent predictors of BMD in all regions of the proximal femur. Estrogen metabolism may modulate BMD in men. Increased urinary 16alphaOHE(1) and E(3) levels are associated with high BMD at the proximal femur, and 16alphaOHE(1) appears to be a major determinant of BMD among the metabolites evaluated.