Puberty is a time when the foundation is laid for healthy bone mass. Over the course of puberty, 26% of bone mass is established in the 4-year period of peak height velocity and up to 60% of adult peak bone mass is established. Factors that affect normal bone mineralization include calcium intake, vitamin D status, degree of physical and weight bearing activities, hormones, genetics, body weight, and general health and nutrition status. HIV-infected children, youth, and adults have lower bone mineral density (BMD) than would be expected for healthy people of similar age, weight, and race. As the majority of perinatally HIV-infected U.S. children are entering or in adolescence, the potential for HIV-related impaired BMD during the adolescent peak of bone mass acquisition is of particular concern. The purpose of this study was to compare changes in BMD of the lumbar spine from pre-treatment levels to 24 and 48 weeks after alendronate treatment or placebo in HIV-infected children and adolescents.