Bronchoalveolar lavage (BAL) has proved valuable in the diagnosis of pulmonary complications in immunosuppressed patients. We evaluated the diagnostic yield of BAL in pulmonary complications in bone marrow transplantation (BMT) recipients. We reviewed sequentially the records of 89 patients during an 18-month period. BAL was diagnostic in 42 patients (47%). The most common pulmonary complication diagnosed by BAL was diffuse alveolar hemorrhage (n = 15); followed by bacterial pneumonia (n = 10), respiratory syncytial virus (n = 8), aspergillosis (n = 6), Pneumocystis carinii pneumonia (n = 5), cytomegalovirus (CMV) (n = 4), and others (n = 4). The final diagnoses in the BAL non-diagnostic group were: bacterial pneumonia (n = 6), CMV (n = 6), idiopathic pneumonia syndrome (n = 5), cancer recurrence (n = 4), cardiogenic pulmonary edema (n = 4), and others (n = 9). We conclude that BAL is a useful diagnostic tool in BMT-related pulmonary complications.