To determine whether the failure to attain normal bone mass in young adulthood contributes to the later development of osteoporotic fractures. Design: Case-control study. Setting: Referral-based bone clinic at a large teaching hospital. Patients: Sequential sample of 35 asymptomatic relatives, aged 19 to 59 years, of patients with osteoporotic fractures, and 24 patients with osteoporotic fractures. Conclsuions: Mean bone mass is lower in apparently healthy young and middle-aged adult relatives of osteoporotic patients than in normal persons with no family history of osteoporosis. Our findings suggest that the failure to attain an adequate peak bone mass may play an important role in the later development of osteoporotic fractures. Relatives of osteoporotic patients should be advised to have measurements of bone mass taken. This measurement should be taken at the spine, because peripheral sites do not appear to provide adequate information about early osteoporosis.