The histological changes in 17 biopsy specimens of the lung showing graft-versus-host disease (GVHD) in bone marrow transplant (BMT) recipients were reviewed and correlated with the patients' clinical courses. These morphological changes fell into four transplant-related categories: diffuse alveolar damage, lymphocytic bronchitis/bronchiolitis with interstitial pneumonitis, bronchiolitis obliterans organizing pneumonia, and cicatricial bronchiolitis obliterans. Pulmonary disease correlated with the presence of GVHD at extrathoracic sites. Patients with active lymphocytic bronchitis/bronchiolitis, cicatricial bronchiolitis obliterans, and diffuse alveolar damage had particularly poor outcomes. A proposal is offered for the categorization of the pulmonary damage caused by GVHD in BMT recipients.