Fifty-two episodes of fever and new pulmonary infiltrates were evaluated prospectively in 51 renal allograft recipients. Thirty-nine flexible fiberoptic bronchoscopies were performed in the diagnostic evaluation of these infiltrates. Specific etiologic diagnoses were obtained in 30 (77%) of the patients. This information was clinically useful, as defined by preset criteria, in 21 (54%) of the patients and definitive but not clinically useful in an additional 9 (23%). In the remaining 9, it was neither definitive nor clinically useful. Microbiology brush specimens were useful in establishing etiologic diagnoses in 12 (44%) of the 27 patients in whom it was performed. Transbronchial lung biopsies yielded specific etiologic diagnoses in 9 (53%) of the 17 biopsies obtained. Complications related to the bronchoscopic procedure occurred in 2 patients (5% of total bronchoscopies). No prolonged morbidity was noted. We conclude that fiberoptic bronchoscopy is a safe, useful procedure, and should be considered early in the diagnostic evaluation of pulmonary infections in renal transplant recipients.