Despite accumulating data in adults, little information is available regarding the effects of HIV infection and antiretroviral therapy on body composition in children. Preliminary information indicates that lean body mass is lost in preference to fat mass in HIV-infected children, supporting the theory that failure to thrive in HIV infection is often cytokine mediated. It can be hypothesized that changes in body composition (lean body mass) may predict changes in weight growth velocity and may give an early clinical indication of treatment failure. If so, body composition measurement may yield an additional outcome measure for clinical trials, equivalent in utility to other laboratory measures of treatment response, e.g., persistent CD4+ cell count changes. Additionally, if body composition changes are highly correlated with responses in viral load, body composition may prove to be a more affordable measure of antiretroviral effectiveness in developing countries.