For 89 human immunodeficiency virus (HIV)-positive and 32 HIV-negative immunocompromised patients who had 121 episodes of Pneumocystis carinii pneumonia (PCP), clinical features and changes over time were compared. HIV-infected patients characteristically had a longer duration of symptoms (23 vs. 13 days; P<.005); were younger (39 vs. 48 years; P<.001); had a higher frequency of sweating, weight loss, and thoracic pain; and had fewer admissions to the intensive care unit (16% vs. 31%; P<.05). In addition, they had significantly higher hemoglobin levels, lower thrombocyte counts, lower C-reactive protein values, and a higher proportion of eosinophils and lymphocytes in bronchoalveolar lavage fluid. After 1995, HIV-negative patients' mean length of stay dropped from 34 days to 16 days (P<.005), and their hospital mortality rate dropped from 29% to 7% (P<.001). HIV-positive patients with PCP differed in several aspects from those without HIV infection. Knowledge gained from experience with treatment of opportunistic infections in patients with AIDS has improved the management of PCP in patients with other immunodeficiencies.