Little is known of pulmonary function in survivors of acute lymphoblastic leukaemia (A.L.L.); this is despite the fact that some drugs used, most notably methotrexate, have well-recorded pulmonary toxicity, and the most common infections during therapy in most series are of upper and lower respiratory tract. As part of a survey of all cancer survivors attending the Royal Hospital for Sick Children in Edinburgh, 38 leukaemic patients, who had completed treatment 3 months to 14 years 6 months (median 6 years and 8 months) prior to survey were assessed with regards to their respiratory status. Each patient completed a questionnaire and had spirometry and lung volumes measured; 30 patients additionally had transfer factor for carbon monoxide (TCO) measured. There were 21 children, 11 adults, and 6 patients in the age range between child and adult. Of the 26 adults and children studied with complete data available, 17 (65%) had one or more low values for vital capacity (VC), total lung capacity (TLC), residual volume, or TCO. Mean VC, TLC and TCO were significantly lower than the mean of the predicted values (P less than .001). Gas transfer per unit lung volume (KCO) was normal in all cases. Few patients had symptoms of respiratory disease. There was an increased incidence of low TCO in patients diagnosed under 8 years of age. Impairment of lung growth could be a contributing factor to the observed abnormalities in pulmonary function. Impairment of pulmonary function in survivors of A.L.L. may be of significance for them in later life.