We prospectively followed a well characterized cohort of patients post-bone marrow transplantation for changes in pulmonary function. Thirty-four recipients without respiratory symptoms were available for follow up with a mean of two years. Spirometry and other measures of lung volume were well preserved following bone marrow transplantation. A progressive 11.9 percent decline in percent predicted diffusing capacity per year occurred. Age, cigarette smoking, type of cytoreductive therapy, type of GVHD prophylaxis, and the occurrence of AGVHD did not affect longitudinal changes in pulmonary function. Patients receiving transplants for CML developed a highly significant fall in diffusing capacity. Asymptomatic patients with CGVHD developed evidence of progressive obstructive ventilatory impairment. This suggests a subclinical spectrum of patients who may progress to the development of bronchiolitis obliterans and respiratory failure post-bone marrow transplantation.