The World Health Organization (WHO) classification of bone mineral density is based on population studies in postmenopausal women. However, the increased use of bone densitometry has raised questions regarding the applicability of this classification to men, premenopausal women, and children. These questions were addressed at the International Society for Clinical Densitometry 2003 Position Development Conference. T-scores can be used and a diagnosis of osteoporosis made for T-scores of -2.5 or less (male reference database) in men age 65 yr and older and in men from 50 to 64 yr of age if other risk factors for fracture are present. The WHO classification should not be applied to premenopausal women. Men and premenopausal women with secondary causes of low bone density or bone loss may be clinically diagnosed with osteoporosis, taking bone density into consideration. In children and adolescents (males and females less than 20 yr of age), there are no densitometric criteria for diagnosing osteoporosis. If Z-scores are -2.0 or less (using pediatric databases of age-matched controls), then a characterization such as "low bone density for chronologic age" is appropriate. In men under 50 yr of age, premenopausal women, and children, Z-scores, not T-scores, should be used when reporting bone density results.