We evaluated 39 episodes (in 32 patients) of pulmonary parenchymal infiltrates following cardiac transplantation with fiberoptic bronchoscopy (FOB) in a prospective study of 94 consecutive recipients. Initial FOB established the diagnosis in 24/39 (62 percent) instances. Subsequent examinations included repeat FOB (five), open lung biopsy (five), needle aspiration (two), and autopsy (nine), establishing 49 diagnoses. Specific pathogens were identified in 45 instances, neoplasm in two, and idiopathic interstitial pneumonitis in two. Bronchoalveolar lavage alone yielded diagnoses in 63 percent and transbronchial biopsy and bronchial washings/brushings in 46 and 43 percent, respectively. Transbronchial biopsy suggested idiopathic interstitial pneumonitis in 17 instances, but four had spontaneous clearing, and open lung biopsy or autopsy showed alternative diagnoses (particularly CMV and Aspergillus) in 11. The main complication of FOB was moderate (25 to 100 ml) hemorrhage after transbronchial biopsy (10 percent); no severe episodes occurred despite elevated pulmonary vascular pressures. In this population of immunocompromised hosts: (1) bronchoalveolar lavage is the most sensitive bronchoscopic technique for detecting infection; (2) transbronchial biopsy is not useful in detecting CMV or Aspergillus infection; (3) pulmonary hypertension is associated with some risk of moderate but not severe hemorrhage after transbronchial biopsy.