Osteoporosis is a major health problem for men and women. Although women are disproportionately affected, the aging of the population and the exponential fracture increase in older persons ensure that the absolute numbers of osteoporotic fractures in both sexes will increase strikingly by the end of this decade. Important sex-specific differences are already known to occur in bone physiology and geometry, fracture epidemiology, bone gonadal hormone response, and post-hip-fracture mortality. These differences point to the importance of doing separate studies of osteoporosis in males and females and will hopefully lead to more effective and prevention-based strategies in the future.