Bronchiolitis obliterans has emerged as the major long-term complication of heart-lung transplantation. We reviewed the histologic findings in lungs obtained from 11 patients who had received a combined heart-lung transplant at The Johns Hopkins Hospital. Ten lungs were obtained at autopsy, and one was obtained from a patient who was retransplanted because of severe bronchiolitis obliterans. Bronchiolitis obliterans was identified in seven of these 11 lungs. Three of the seven lungs with bronchiolitis obliterans were from patients who had received their transplants more than 6 months previously; the bronchiolitis obliterans in these patients was characterized by a relatively acellular concentric fibrosing process that was limited to the terminal bronchioles. The bronchiolitis obliterans in these three patients was felt to be secondary to chronic lung allograft rejection. Four of the seven patients with bronchiolitis obliterans had received their transplants less than 6 months previously; the bronchiolitis obliterans in these patients was focal and cellular. It extended into the distal alveolar spaces and, in several cases, was associated with aspirated material and foreign body-type giant cells. All four of these patients had concurrent infections, aspiration, or large airway obstruction, which were felt to be responsible for the development of bronchiolitis obliterans. Bronchiolitis obliterans in lung allograft recipients may have a variety of etiologies, and the etiology of this process in a particular case can often be deduced by the morphologic appearance of this lesion.