Osteoporosis is a major public health problem, and its prevalence may be increasing. Unfortunately, once spinal fractures occur, the treatment of osteoporosis is less than satisfactory. Prevention is thus the preferred approach. To initiate a prevention program would require identification of those at greatest risk. Usually, small, thin, white or Asian women who have had an early menopause (or oophorectomy) can be considered most at risk. Other factors may include low calcium intake, inactivity, alcohol and cigarette consumption, and nulliparity. For those clinically thought to be at greatest risk, bone mass measurements may be used to more clearly define the population for whom preventive therapy would be indicated. Prevention consists of dietary and lifestyle alterations, primarily increases in calcium intake and exercise, and the judicious use of estrogens. Estrogen therapy remains the single most effective method for prevention of bone loss.