Sixty-one diagnostic biopsies for Pneumocystis carinii pneumonia were performed on 40 homosexual male patients with acquired immunodeficiency syndrome (AIDS), using flexible fiberoptic bronchoscopy. Bronchial brushings and bronchoalveolar lavage were performed in conjunction with the biopsy in 58 and 29 bronchoscopies, respectively. Using a rapid methenamine silver stain, P carinii pneumonia was diagnosed in 27 (68 percent) of the patients. Twenty of these patients had a repeat biopsy one or more times for evaluation of therapy. Eighteen of the biopsies following two to three weeks of therapy were positive. There was an 84 percent correlation between findings on transbronchial brushing and biopsy (89 percent on initial biopsy before treatment) and an 86 percent correlation between bronchoalveolar lavage and biopsy. Additionally, transbronchial brushing permitted demonstration of Pneumocystis organisms in four follow-up bronchoscopies in which the biopsy was negative or inadequate. Rapid methenamine silver stain of transbronchial brushings permits diagnosis of P carinii pneumonia in patients with AIDS within one-half hour of bronchoscopy.