Osteoporosis can occur at any age. In premenopausal osteoporosis, full achievement of peak bone mass may be curtailed, and accelerated bone loss may occur in young adulthood. Premenopausal osteoporosis may be associated with chronic glucocorticoid therapy, prolonged amenorrhea, anorexia nervosa, rheumatoid arthritis, and diseases that affect calcium and vitamin D metabolism. Lesser degrees of bone loss may be associated with common conditions such as dieting, low calcium intake, smoking, and oligomenorrhea. Owing to a paucity of prospective studies on screening and treatment in younger age groups, few practice recommendations exist to guide the management of osteoporosis in young adults. We review the most important clinical concerns in premenopausal osteoporosis, including measurement of bone mass, normal bone accrual, risk factors for premature bone loss, clinical outcomes, and management issues. We emphasize clinically relevant information for primary care physicians, who are usually the first to encounter premenopausal patients with risk factors for early bone loss.