We prospectively studied causes of fever in patients with human immunodeficiency virus (HIV) infection that required admission to a municipal hospital. A total of 168 HIV-infected persons were admitted for 220 episodes of fever: 72% were male, 80% were nonwhite, 65% reported prior injection drug use, and 74% had a baseline CD4 lymphocyte count of < 200/mm3. Bacterial infections, principally pneumonia, accounted for > 60% of the episodes; Streptococcus pneumoniae and Staphylococcus aureus were most commonly isolated. Pneumocystis carinii pneumonia (PCP) and disseminated infection with Mycobacterium avium complex (MAC) comprised 53% of the remaining sources of fever. In comparison with episodes of fever due to nonbacterial causes, those associated with common bacterial infections were significantly more likely to involve patients with a history of injection drug use (P = .02), higher admission leukocyte count (P < .004), shorter duration of fever (P = .003), shorter hospital stays (P = .0001), and a CD4 count of > 100/mm3 (P = .002). We conclude that bacterial infection, especially pneumonia, is a common cause of fever in HIV-infected patients admitted to our hospital. Patients with bacterial infections are more likely to report a history of injection drug use and have CD4 counts of > 100/mm3, shorter duration of fever, decreased length of hospitalization, and lower mortality than patients with fever due to PCP, disseminated MAC infection, or other causes.