To assess the correlation between the remaining serum testosterone and bone mineral density(BMD), and to determine the effect of exogenous testosterone on BMD in subjects with male hypogonadism, we evaluated the serum testosterone levels and BMDs of the femur neck, Ward's triangle and the spine(L1-4) in 20 subjects with Klinefelter's syndrome and 7 with hypogonadotropic hypogonadism before and after testosterone replacement. BMDs of the femur neck, Ward's triangle and the spine were below the age-matched normal mean at 77.8%(21/20), 74.1%(20/27) and 88.9%(24/27), respectively. There were significant differences in serum testosterone levels and the spinal BMD between the two groups and the BMD of the spine closely correlated with the serum testosterone level (R = 0.63, p < 0.001). Following a mean 11.8 ± 4.9 months of testosterone replacement, the BMD at all sites increased significantly and the pretreatment difference in spinal BMD between the two groups disappeared. We conclude that, although testosterone may increases the bone density, it has a site-specific effect of maintaining and increasing the bone mass especially at the spine in male hypogonadism.