Weight loss is a negative prognostic indicator in patients infected with HIV. Mortality rates rise measurably with as little as 3-5% weight loss over 6 months. The sensitivity of this measure is at least partly due to the correlation between weight loss and a metabolic cachexia that has been observed with other infections, trauma, and some cancers. However, the cachexia in patients with HIV, commonly termed wasting, may also be due to, or exacerbated by, reduced caloric intake, gastrointestinal dysfunction, or metabolic abnormalities independent of abnormal energy expenditure. In patients with HIV wasting, therapies should be directed both at reversing the underlying source of protein energy malnutrition and at other factors that may be contributing to weight loss.