The infectious pathogens and associated cellular reactions in 75 pulmonary cytological specimens obtained largely by fiberoptic bronchoscopy from 36 patients with the acquired immune deficiency syndrome are described and correlated with the biopsy specimens. An opportunistic pathogen was diagnosed in 33% of cytological preparations. Pneumocystis carinii was encountered most frequently; Cryptococcus neoformans and cytomegalovirus were also seen. The polymorphonuclear neutrophil was the predominant inflammatory cell. Cells of secondary import were bronchial cells and type II pneumocytes with atypia and hyperplasia. Bronchoalveolar lavage had the highest yield, positive predictive value, and sensitivity for all pathogens and for P. carinii. An opportunistic pathogen was diagnosed in 69% of patients utilizing histological material and in 33% using cytological samples; the rate of diagnosis increased to 72% when the samples were combined.