Androgens have the unique feature that they may activate the androgen receptor but also function as a prohormone for estrogens. 20 Indeed, it has been established that, in some target tissues, like the brain, the conversion of testosterone into 17βestradiol by P450 aromatase is essential for masculinization. 20 Aromatase26 as well as estrogen9 and androgen7 receptors are present in human osteoblasts, and androgen receptors have also been demonstrated in human osteocytes and bone marrow mononuclear cells of both men and women1 and in murine stromal marrow cells. 3 Furthermore, not only the aromatizable androgen, testosterone, and the nonaromatizable androgen, 5α-dihydrotestosterone (DHT), but also 17β-estradiol, prevent bone loss in male orchidectomized rats. 38 However, activity of 5α-reductase, the enzyme that converts testosterone into DHT, was demonstrated in human bone tissue27 but not confirmed in cultured rat periosteal cells. 32 Androgens may therefore affect bone metabolism either directly by stimulation of the androgen receptor or by their conversion in estrogens and stimulation of the estrogen receptor (Figure 1).