To assess the effect of testosterone deficiency on skeletal integrity in men, we determined bone density in 23 hypogonadal men with isolated gonadotropin-releasing hormone deficiency and compared those values with ones from controls. Cortical bone density, as assessed by single-photon absorptiometry of the nondominant radius, ranged from 0.57 to 0.86 g/cm² (mean ± SE, 0.71 ± 0.02) in patients with fused epiphyses and from 0.57 to 0.67 g/cm² (mean, 0.61 ± 0.01) in patients with open epiphyses, both of which were significantly (p < 0.001) lower than normal. Spinal trabecular bone density, as assessed by computed tomography, was similarly decreased (p < 0.0001) and ranged from 42 to 177 mg K₂HPO₄/cm³ (mean, 112 ± 7). Cortical bone density was at least 2 SD below normal in 16 of 23 men, and 8 men had spinal bone densities below the fracture threshold of 80 to 100 mg K₂HPO₄/cm³. Osteopenia was equally severe in men with immature and mature bone ages, suggesting that abnormal bone development plays an important role in the osteopenia of men with idiopathic hypogonadotropic hypogonadism.