Abstract Weak and osteoporotic bones in old age are an increasing cause of mortality and painful physical impairment of the elderly, especially in the western world. Bone mineral accrual during childhood and adolescence is thought to play a vital role in preventing osteoporosis. Identifying and optimizing the factors influencing peak bone mass is thus important for the prevention of osteoporosis and related fractures. A main aim of this thesis was to investigate the potential effects of various types of weight-bearing physical activity on bone accretion in young males just out of puberty. The results from our subgroups of athletes consisting of badminton, ice hockey, and soccer players suggest that weight-bearing physical activity gives rise to regional specific bone response that is determined by the degree of impact of the activity in areas subject to mechanical loading (papers I–IV). In summary, the bone is sensitive to loading after puberty in males, and important bone mass gains can be achieved by proper amount and type of exercise. Another aim of this thesis was to studythe effect of detraining on weight-bearing and non-weight-bearing bone in a cohort of adolescent males who participated in ice hockey and soccer training. Our results indicate that exercise-induced bone mineral density benefits decline, predominantly in weight-bearing bones, after retirement from an active sports career (papers II–IV). High bone density stemming from physical loading might be at least partly preserved even by reduced physical activity at nonweight-bearing sites after about three years of reduced activity (III, IV). A final aim was to follow prospectively the development of BMD during years of reduced activity in former male athletes, and evaluate whether exercise during adolescence could be associated with fewer fractures in old age. We found fewer fragility fractures in a cohort of 400 former athletes compared to in 800 age-matched controls. Thus, high bone density stemming from previous weight-bearing physical activity may reduce the risk of sustaining fragility fractures in the elderly. Key words: physical activity, peak bone mineral density, males.