Pneumocystis carinii pneumonia has emerged as a significant cause of morbidity and mortality in immunocompromised patients with and without AIDS. To determine differences in P. carinii pneumonia in patients with and without AIDS, the P. carinii parasite numbers, lung inflammatory cell populations, gas exchange, and survival were assessed in a series of 75 consecutive patients with P. carinii pneumonia. Bronchoalveolar lavage was used to quantify the parasite and inflammatory cell numbers in these patients. The data from this study indicate: (1) patients with P. carinii pneumonia and AIDS have significantly greater numbers of P. carinii per ml of lavage compared to other immunocompromised patients with P. carinii pneumonia (p less than 0.0001); (2) patients with P. carinii pneumonia and AIDS also have significantly fewer neutrophils recovered in the lavage compared to other immunocompromised patients with P. carinii pneumonia (p = 0.0001); (3) patients with AIDS and P. carinii pneumonia have higher arterial oxygen tensions than those patients with P. carinii pneumonia in conditions other than AIDS (p = 0.008); and (4) increased lavage neutrophils (rather than parasite number) correlate with poorer oxygenation and poorer patient survival (p = 0.01). This investigation demonstrates substantial differences in lung inflammation and parasite number during P. carinii pneumonia in patients with and without AIDS. The data further suggest that lung inflammation contributes substantially to respiratory impairment in patients with P. carinii pneumonia.