In order to investigate the incidence of pulmonary function complications following bone marrow transplantation (BMT), 17 patients with leukaemia and 8 with aplastic anaemia were sequentially assessed over a one year period. Before BMT, all the patients were free of respiratory symptoms and had both normal chest X-ray and routine lung function tests. However, 5 patients disclosed airway hyperreactivity. Aplastic anaemia patients had significantly lower haemoglobin-adjusted diffusing capacity for carbon monoxide (DLCO) than those with leukaemia, a finding significantly related to the lower haemoglobin values shown in the former individuals. Following BMT there were transient mild to moderate reductions in DLCO and static lung volumes; moreover, patients with leukaemia had lower DLCO than those with aplastic anaemia. Fourteen of the 25 patients had ventilatory defects, including 10 individuals with bronchial hyperresponsiveness. Post-BMT lung function changes were transiently accompanied by mild to moderate symptoms of respiratory disease in most of the patients.