The World Health Organization criteria for classification of bone mineral density (BMD) cannot be applied to premenopausal women because the relationship between BMD and fracture risk is not the same as in postmenopausal women. Approximately 2.5% of premenopausal women have BMD that is more than 2.0 standard deviations below the mean BMD of an age-, gender-, and ethnicity-matched reference population. Most premenopausal women with low BMD have low peak bone mass and low 5- to 10-year probability of fracture. The management of these patients involves nonpharmacologic lifestyle measures and reassurances that fracture risk is low. A minority of premenopausal women with low BMD have significant elevation of fracture risk, usually a result of contributing diseases, conditions, or medications that may be identified and treated. Premenopausal women with fractures are at increased risk for postmenopausal osteoporosis and fractures later in life.