Osteoporosis in men has become better recognized as an important health problem, particularly in the aged. Nevertheless, the knowledge available remains limited and fragmentary. Many clinical decisions must be based on extrapolations from the more complete understanding of osteoporosis in women. Gender differences in the causation and manifestation of the disease are probable, and the following issues clearly require better definition: Pathomechanisms. What are the determinants of age-related bone loss and fractures? What is the nature of idiopathic osteoporosis? How do other medical conditions result in osteoporosis? Cost-effective strategies for disease detection. Strategies for the prevention and treatment of bone loss and for the prevention of fractures.