SummaryTwo important determinants of bone density should be considered when establishing strategies for preventing osteoporosis: peak bone mass and subsequent rate of bone loss. Peak bone mass is both genetically determined and influenced by environmental factors such as exercise and dietary calcium. The benefit of exercise is mitigated if it is not weight-bearing, or if it reduces oestrogen production by delaying the menarche or disturbing menstrual function.Oestrogen deficiency is the major determinant of bone loss in perimenopausal women. However, weight-bearing exercise and high dietary calcium appear to limit bone loss during the first 5 to 10 years after the menopause. Prescribing for the prevention of osteoporosis should include individual advice concerning the patient's lifestyle.