Conditioning with busulphan (BU) and cyclophosphamide (CY) prior to allogeneic bone marrow transplantation (BMT) is an alternative to regimens that include total body irradiation (TBI). The aim of the study was to assess the occurrence and degree of lung function impairment after this treatment. Prospectively, 43 consecutive patients, aged 17-51 (median 31) yrs, were examined by lung function measurements and clinical and radiographic evaluation, prior to BMT and at 3 month intervals up to 1 yr after BMT. All patients had normal chest radiographs before BMT and at the 12 month follow-up. Mean baseline values were above 100% of predicted normal for lung volumes and above 90% for gas transfer. Excluded from the lung function follow-up analyses were nine patients who had suffered infectious pneumonia and/or developed obliterative bronchiolitis. For the remaining patients (n = 34), mean alveolar volume (VA), forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) had dropped by nearly 10% compared with baseline 3 months after BMT, but were restored within 1 yr. FEV1/FVC x 100 (FEV1%) was increased, reflecting the restrictive pattern. Hb-adjusted transfer factor of the lungs for carbon monoxide (TL,CO) had dropped by 20% after 3 months, and remained reduced by 15% after 1 year. Prior to BMT the smokers had significantly lower TL,CO than the nonsmokers, and after BMT the difference was accentuated. Reductions in lung function were independent of sex, age and type of haematological disorder. We conclude that BMT with BU/CY is associated with transient declines in lung volumes and a persistent reduction in gas transfer 1 yr after therapy.(ABSTRACT TRUNCATED AT 250 WORDS)