Osteoporosis in men is a common disorder, the incidence of which is increasing. Bone is lost with advancing age in men as well as in women, leading to an increased risk of fracture after minimal trauma. Recent epidemiological data from North America suggests a lifetime risk of fracture of the hip, spine, or forearm of 13.1% in white men aged 50 [1]. Despite the considerable public health burden attributable to these fractures, our understanding of their pathogenesis is incomplete and there is no established treatment for osteoporosis in men.Androgen deficiency is thought to play an important role in many cases of male osteoporosis. Biochemical evidence of hypogonadism is found in about 20% of men with vertebral fractures [2] and up to 50% of men with hip fractures [3,4], often without other clinical features of gonadal failure. In cell culture studies using human osteoblastlike cells from both men and women [5][6][7], specific androgen receptors can be identified by nuclear and cytosolic binding assays (Table 1). Similarly, osteoblasts obtained from fresh femoral bone specimens can also metabolize the circulating androgen androstenedione to testosterone and 5␣-dihydrotestosterone (DHT) in both men and women [8] at rates dependent mainly on androstenedione concentration. Cells of osteoclast lineage may also have these properties, though this is less well established. Androgen receptors have also been directly demonstrated in osteoblasts and osteocytes around the growth plate [9]. Androgens may promote proliferation and differentiation of osteoblasts, inhibit osteoclast recruitment or affect osteoblast-to-osteoclast signaling.Animal studies have mostly been performed in orchiectomized (orch) male rats, though there are reservations about the appropriateness of this model because the rat skeleton grows throughout life. Replacement therapy with either testosterone or 5␣-DHT results in enhanced bone mass, though this remains lower than in control rats [10].In male humans, androgens have a marked effect on bone growth and peak bone mass via the growth hormone/ IGF-I axis; accordingly, androgen deficiency before puberty leads to lower bone mineral density (BMD) in both cortical and trabecular bone. Loss of androgens in later life is associated with increased bone resorption and loss of trabecular bone, but cortical bone mass is not greatly affected [11,12]. In a study of 12 men who had undergone judicial castration for ''sexual delinquency'' [13], bone turnover was increased and lumbar spine BMD fell significantly, with evidence of the most rapid loss in the first 5 years after castration. Rates of bone loss were as high as 7% per year, comparable to loss rates seen in early postmenopausal women. In this study, calcitonin therapy was effective in reducing bone loss, and low calcitonin levels have been reported in another group of young hypogonadal men who had not undergone surgery [14]. Others have found low plasma 1,25 dihydroxyvitamin D levels and impaired calcium absorption in hypogonadal men [15].This combination o...