The survival from acute lymphoblastic leukaemia in childhood is now approximately 60-70%, and from acute myeloid leukaemia, up to 50%. However, there is little information on the effects of intensive chemotherapy and radiotherapy used in the treatment of these conditions on lung function and exercise capacity in the long term. Severity survivors of acute leukaemia from one centre in the UK were studied. Measurements of lung volumes, spirometry and transfer factor were made. Each child also performed a standard, symptom-limited maximal exercise test on a cycle ergometer. Predictive equations for indices of lung function and exercise tolerance were calculated from 146 age- and sex-matched control subjects. The results of the survivors of leukaemia were compared to these. There was a significant reduction of forced expiratory volume in one second (FEV1), forced vital capacity (FVC), total lung capacity (TLC), and transfer for carbon monoxide (DLCO; P < 0.05 for each measurement), in the survivors of leukemia when compared to the control subjects. In addition, there was a mild but significant reduction of both maximal and submaximal indices of exercise capacity in the leukaemic group. A multivariate analysis was carried out to identify those variables acting independently to reduce lung volumes. For FEV1, FVC and TLC, these were craniospinal irradiation, cyclophosphamide and chest complications during treatment. For a reduction in DLCO, the significant factors were administration of anthracyclines, craniospinal irradiation and bone marrow transplantation. Survivors of acute leukemia have impaired pulmonary function and exercise capacity. Long-term cardiopulmonary follow-up may be necessary and new regimens devised which reduce long-term toxicity without compromising survival rates.