In this study of 28 immunocompromised patients, it was found that Pneumocystis carinii pneumonia could be easily and reliably diagnosed by examination of routinely prepared, Papanicolaou-stained cellular samples obtained by bronchoalveolar lavage, bronchial brushing, and bronchial washing. The distinctive intra-alveolar exudate of pneumocystosis observed in lung biopsy specimens was readily discernible in all of the cellular samples that demonstrated P. carinii by special stains. The exudate was not present in any of the P. carinii-negative samples. Routinely prepared, Papanicolaou-stained cellular samples can be relied upon for the rapid diagnosis of P. carinii pneumonia.