BACKGROUND Poor nutritional status is associated with immunologic impairment and adverse health outcomes among adults infected with human immunodeficiency virus (HIV). METHODS We investigated body mass index (BMI), middle upper arm circumference (MUAC), and hemoglobin (Hgb) concentrations at initiation of antiretroviral therapy (ART) in 18,271 HIV-infected Tanzanian adults and their changes in the first 3 months of ART, in relation to the subsequent risk of death. RESULTS Lower BMI, MUAC, and Hgb concentrations at ART initiation were strongly associated with a higher risk of death within 3 months. Among patients who survived >3 months after ART initiation, those with a decrease in weight, MUAC, or Hgb concentrations by 3 months had a higher risk of death during the first year. After 1 year, only a decrease in MUAC by 3 months after ART initiation was associated with a higher risk of death. Weight loss was associated with a higher risk of death across all levels of baseline BMI, with the highest risk observed among patients with BMI <17 kg/m(2) (relative risk, 7.9; 95% confidence interval, 4.4-14.4). CONCLUSIONS Poor nutritional status at ART initiation and decreased nutritional status in the first 3 months of ART were strong independent predictors of mortality. The role of nutritional interventions as adjunct therapies to ART merits further investigation.