To assess the sensitivity of bronchoalveolar lavage in patients with the acquired immunodeficiency syndrome (AIDS) in diagnosing Pneumocystis carinii pneumonia (PCP), we prospectively performed 27 bronchoalveolar lavages (BAL) in 16 patients either because there was an initially high index of suspicion of PCP or in order to assess therapeutic response to anti-Pneumocystis medication in those patients who had had PCP documented. Pneumocystis organisms were demonstrated on BAL specimens in 16 of 18 procedures in patients with histologic evidence of PCP on simultaneously obtained pulmonary tissue. The diagnosis was established rapidly by BAL and there was no substantial morbidity attributable to the procedure. Subsegmental BAL may be an important and sensitive tool for early diagnosis of PCP in patients with AIDS.