Fifty-two bronchoalveolar lavages (BAL) were performed in order to investigate 46 episodes of pneumonitis that occurred after allogeneic bone-marrow transplantation. No complications have been attributed to this procedure. A specific etiologic diagnosis was obtained in 24 of 46 episodes (52%) by 26 of 52 BAL (50%). Cytomegalovirus (CMV), diagnosed by the presence of typical inclusions, was the pathogen most frequently identified by BAL (13 of 46 episodes) and was associated with other causes of pneumonia in 4 patients. The other causes of pneumonitis diagnosed by BAL were: giant-cell pneumonia: 1, aspergillosis alone: 3, Pneumocystis carinii: 1, Hemophilus influenzae: 3, isolated pulmonary hemorrhage: 3. One false negative (aspergillosis, n = 1) was diagnosed at autopsy. The overall mortality rate of these episodes was 24%. Thus, BAL appears to be a rapid and reproducible diagnostic method for monitoring pneumonitis in grafted patients, particularly CMV pneumonitis, and may avoid the need for surgical biopsy.