The clinical course, serial pulmonary function studies, lung histopathologic findings, and treatment in two patients after bone marrow transplantation for acute monoblastic leukemia or aplastic anemia are presented. The course in one patient has been slowly progressive for 2 years and characterized by chronic obstructive airways disease and recurrent pneumothoraces. Histopathologic changes were nonspecific, characterized by chronic interstitial pneumonitis and interstitial fibrosis. In the second patient there was insidious onset of disease with increasing dyspnea on exertion and rapid clinical deterioration; he died within 4 months of severe obstructive airways disease. Necrotizing bronchitis and bronchiolitis characterized the lung findings. Neither patient responded to conventional bronchodilator therapy, and prednisone was the only agent to produce subjective, though transient, improvement. Symptomatic obstructive airways disease associated with chronic graft-versus-host disease is emerging as a potentially major cause of morbidity and mortality after marrow transplantation.