To further improve the diagnostic value of bronchoscopy in the immunosuppressed population presenting with diffuse pulmonary infiltrates, we prospectively investigated the utility of bilateral bronchoalveolar lavage (BAL). We performed 62 bronchoscopies on 52 immunosuppressed patients. Of the 52 patients, 33 had pulmonary infections. The yield for Pneumocystis carinii pneumonia on bilateral BAL was 94 percent (31/33), compared to the 84 percent (51/61) previously obtained with unilateral BAL in our institution. The recovery of P carinii was unilateral in four of five patients without AIDS and in four of 26 patients with AIDS. Transbronchial biopsy gave a yield of 85 percent (11/13). In ten patients with definitive cytomegalovirus (CMV) pneumonia, recovery of CMV by combined culture and cytology was 100 percent. Of nine bronchoscopies with positive cytology for CMV, five showed cytopathologic changes in the BAL from both sides and four in the BAL from one side only. No complications were seen in the 14 patients with thrombocytopenia or the five patients receiving mechanical ventilation. Our findings indicate that bilateral BAL significantly increases the yield for recovery of P carinii (p less than 0.02) and CMV (p less than 0.001) in immunosuppressed patients.