Despite major advances in the treatment and survival of patients infected with human immunodeficiency virus (HIV), weight loss and wasting remain common problems. In the HIV-infected population, weight loss is associated with lower CD4+ cell counts and is an independent predictor of mortality. The etiology of weight loss and wasting is complex and multifactorial. We discuss, on the basis of a large longitudinal cohort that examined nutritional status in HIV infection, data on weight loss and wasting from the present clinical era. The definition, prevalence, and significance of HIV-associated weight loss and wasting are summarized. The etiology of weight loss is discussed for 2 main categories: inadequate nutrient intake and altered metabolism. Finally, studies of interventions to treat HIV-associated weight loss and wasting are discussed. This information is intended to raise awareness among health care providers of HIV-infected patients that weight loss and wasting remain important acquired immunodeficiency syndrome-defining conditions, despite the advent of HAART.