Thirty cases of a first episode of Pneumocystis carinii pneumonia in patients with HIV infection were collected in a 32 month period. Most patients had long standing fever, cough and dyspnea. Laboratory findings were nonspecific. Remarkably, LDH activity was high in 88% of patients and the T4 lymphocyte count was lower than 200/mm3 in all patients in whom it was measured. Chest radiogram showed bilateral alveolar-interstitial pattern in 90% of cases. Bronchoalveolar lavage with ultracentrifugation was found to be the most effective diagnostic study, with 95% sensitivity. The frequency of secondary effects to cotrimoxazole which required to change to pentamidine was 13.3%. During hospital admission, 16.6% of the patients died, and the survivors had mortality rates of 4% and 85% after 3 and 20 months, respectively.