We have previously demonstrated that estrogen metabolism is one of the determinants of bone density after menopause. Increased hydroxylation to relatively nonestrogenic metabolites 2-hydroxyestrone (2OHE1) and 2-methoxyestrone (2MeOE1) was associated with low bone mineral density (BMD), while increased hydroxylation to the potent 16alpha-hydroxyestrone (16alphaOHE1) and weakly estrogenic estriol (E3) was associated with higher BMD. In this study, we tested the hypothesis that response to estrogen-hormone replacement therapy (ERT/HRT) is also related to individual differences in estrogen metabolism. Urinary estrogen metabolites were measured in 310 postmenopausal women using ESTRAMET enzyme immunoassay kit. Of these, 163 were on HRT with conjugated equine estrogen (CEE) and medroxyprogesterone acetate (MPA, Premarin and Provera) or ERT with conjugated equine estrogen alone (Premarin), and 147 women not on ERT/HRT acted as comparison. Annual rates of BMD changes were calculated on a subset of 81 women on ERT/HRT who had more than one previous BMD measured by dual-energy X-ray absorptiometry (DEXA). Controlling for age, years since menopause (YSM), body mass index (BMI), waist to hip ratio, and smoking, we found that urinary estrogen metabolite levels were significantly higher in ERT/HRT-treated women compared to those not on ERT/HRT. Furthermore, women in the higher 2 tertiles of 2OHE1 and 2OHE1/16áOHE1 ratio had positive increments in BMD compared to those in the lowest tertile who lost bone while on ERT/HRT. Thus, women with estrogen metabolism favoring the 2-hydroxylation pathway respond favorably to ERT/HRT.