Undernutrition is a frequent complication of evolutive and chronic HIV (human immunodeficiency virus) infection characterized by bodyweight loss and changes in body composition. The Centers for Disease Control and Prevention define AIDS wasting as involuntary loss of more than 10% of body weight, plus more than 30 days of either diarrhea, or weakness and fever. Wasting syndrome has been considered as a case definition of the AIDS disease since 1987. Wasting syndrome is clearly linked to disease progression and death. Despite the progress under the era of highly active antiretroviral therapy (HAART), wasting is still a problem for people with AIDS. A small part of the weight lost is fat. More important is the loss of "lean body mass", which is mostly muscle. Body composition changes during HIV infection are different from those observed in food deprivation. Under the era of HAART, a HIV-associated adipose redistribution syndrome (HARS) was described that associates subcutaneous lipoatrophy and abdominal obesity linked to various metabolic disorders. Several factors contribute to wasting syndrome. Not only low food intake and poor nutrient absorption, but mainly altered metabolism (increased resting energy expenditure) and specific disturbances in protein turnover, which is also increased. Nutritional evaluation of HIV-infected patients should include the measurement of body composition and analysis of nutritional parameters, including albumin, transthyretin and C-reactive protein. Transthyretin seems to be particularly useful to follow the recovery period of malnutrition.