We identified eight patients with bronchiolitis obliterans (BO) in the autopsies of 81 bone marrow transplant (BMT) recipients. Rapidly progressive dyspnoea and cough were the main presenting symptoms in all eight patients, associated with overinflation and/or infiltrative opacity seen on chest X-ray and obstructive disorder revealed by pulmonary function tests. Early lesions were characterized by epithelial loss and an inflammatory infiltrate containing foamy histiocytes with mild luminal narrowing. Partial or total occlusion of the bronchiolar lumina by fibrous connective tissue was the feature of late lesions. Both changes were coexistent in all cases. In one case, small bronchi with cartilage were also affected by the obstructive process, showing bronchitis obliterans. All eight patients showed non-obstructive broncho-bronchiolitis characterized by denuding of respiratory epithelium, mural oedema and an inflammatory infiltrate in addition to BO, and these changes were also seen in 18 patients without BO. The submucosal glands of large bronchi and the trachea showed mucous retention and a mild inflammatory infiltrate in four of the eight patients. Coexistent infectious processes were seen in all cases, cytomegalovirus and Aspergillus being the most frequent organisms. BO probably develops as an immunopathological event related to graft-versus-host disease (GVHD) during the impaired immune status phase of the post-BMT period, possibly initiated by infection. Bronchial gland involvement in chronic GVHD is one of the factors responsible for this abnormal immune status.