Respiratory complications of bone marrow transplantation comprise the majority of its morbidity and mortality. Obstructive airways disease is the most common noninfectious respiratory complication, usually indicative of obliterative bronchiolitis (OB), which occurs in 9% of allogeneic marrow transplant patients. OB is rarely seen after autologous transplant because chronic graft versus host disease (GVH), the most commonly identified risk factor, does not occur in this setting. Alloreactive immunity is likely the cause, with donor type 2 T-helper (TH2) lymphocytes the primary mediators. OB presents at 6 to 12 months post-transplant with cough and dyspnea. Results of investigations include relatively normal or hyperinflated chest radiographs; thickened, dilated airways; and mosaic attenuation on high-resolution computed tomography (HRCT), and fixed airflow obstruction, hyperinflation, and gas trapping on physiological testing. Bronchoscopy and lavage are performed primarily to exclude infections. Transbronchial biopsy is often nondiagnostic, but because the clinical diagnosis is generally sufficient, surgical biopsy is not usually recommended. Histology reveals lymphocytic bronchiolitis, concentric bronchiolar fibrosis, and bronchiolar obliteration. Corticosteroids remain the mainstay of treatment, which is usually required for 3 to 9 months. Response is generally poor, with mortality between 40 and 100%, and lung function infrequently improves. Stabilization with permanent respiratory impairment is common. Early detection and prompt immunosuppression may improve outcomes.