OBJECTIVE The possibility of increased osteoporosis and osteoporotic fractures following therapeutic orchiectomy in men with prostate cancer was investigated. METHODS A total of 235 men with nonstage A prostate cancer diagnosed between 1983 and 1990 was analyzed for therapeutic orchiectomy, other osteoporotic risk factors and subsequent hospital treatment for osteoporotic fractures. The 17 castrated men alive in 1995 were interviewed, and femoral neck bone mineral density was compared to that of 23 controls of similar age. RESULTS Risk factors for osteoporosis, including smoking, slender habitus and atrophic testes, were common among men treated with orchiectomy. Of the men in the study cohort 10 had osteoporotic fractures: 8 of 59 treated with and 2 of 176 without orchiectomy (13.6 versus 1.1%, p < 0.001). First fracture cumulative incidence rates 7 years after castration or diagnosis were 28 and 1%, respectively (p < 0.001). Osteoporotic fractures were much more common than pathological fractures or those due to major trauma (1 each). Bone mineral density averaged 0.91, 0.84, 0.79 and 0.66 gm./cm.2 in 9 controls without prostate cancer, 14 men with prostate cancer before orchiectomy, 9 men at 9 to 60 and 8 men at 60 to 115 months after orchiectomy, respectively. Of the 16 men surviving for longer than 60 months after orchiectomy 6 had osteoporotic fractures, as did 5 of 6 and 5 of 7 with a bone mineral density of less than 0.70 gm./cm.2 and less than 75% of normal for age, respectively. CONCLUSIONS Orchiectomy for prostate cancer is frequently followed by severe osteoporosis, some of which had developed before castration. Appropriate therapy should be identified that does not diminish the antitumorigenic effectiveness of androgen ablation.