Pneumocystis carinii pneumonia was suspected in 52 patients who underwent lung biopsy for diagnosis of diffuse pulmonary infiltrates. All but five of the procedures were limited thoracotomies. There were no instances of significant postoperative hemorrage or of pneumothorax compromising ventilation. The diagnosis of P. carinii pneumonia was made rapidly from imprints of the lung stained by the Gram-Weigert method in the 18 patients who had P. carinii pneumonia. Over-all, the rate of recovery from Pneumocystis pneumonia after treatment with pentamidine isethionate was 41 percent. However, among patients with no other pulmonary lesion (group A), the survival rate was 58 per cent; none of the patients with coexisting pulmonary disease (group B) recovered. The pulmonary infiltrates in the other 32 patients represented a variety of processes, of which 5 were fungal infections and 5 were due to lymphoma. Fifteen patients had an organizing interstitial and intraalveolar pneumonia of undertermined origin.