A 34-year-old obese woman with human immunodeficiency virus (HIV) infection diagnosed a year earlier was seen because of nausea, vomiting, and intermittent diarrhea for 3 weeks. Her current medications included zidovudine. Physical examination revealed tachypnea and tender hepatomegaly. Computed tomography of the abdomen showed hepatomegaly with fatty infiltration. Liver enzymes were within normal range except for elevated lactate dehydrogenase (LDH). The serum bicarbonate value was low, with a lactate level three times normal. The tachypnea and dyspnea worsened as lactate concentrations rapidly increased to 15 times normal. Although her Po2 and cardiac index were initially adequate, the patient had acute respiratory failure. She died with multiorgan dysfunction, including hepatic failure, severe lactic acidemia, disseminated intravascular coagulation, and renal failure. Autopsy revealed hepatomegaly and massive steatosis. Physicians should consider lactic acidosis in patients taking zidovudine and having unexplained tachypnea, dyspnea, and low serum bicarbonate concentrations.