We conducted a prospective study of respiratory function in children undergoing bone marrow transplantation (BMT) for onco-hematological disorders. Each child was evaluated before and 100 days after BMT. The investigations included clinical examination, chest X-ray, and pulmonary function tests (PFT) to determine: slow vital capacity (VC), functional residual capacity (FRC), total lung capacity (TLC), forced expiratory volume in 1 s (FEV1), carbon monoxide diffusing capacity (DLCO), ratio of residual volume (RV) to TLC, and FEV1/VC. The values obtained before and after BMT were compared to predicted values, and the post-BMT values were compared to the pre-BMT values (Student's t-test). From 1986 to 1995, 77 children underwent BMT, of whom 39 were available for testing. The pre-BMT VC (P = 0.0234) and DLCO (P < 0.0001) were lower and FRC higher (P < 0.0001) than predicted values. After BMT, the VC (P = 0.004), TLC (P = 0.044), and FEV1 (P = 0.012) were lower, and the RV/TLC ratio was higher (P = 0.043), compared with pre-BMT data. The observed respiratory abnormalities were not clinically relevant. The only identifiable risk factor for a decrease in lung function was age at BMT. This study shows that some lung dysfunction may be present before BMT and be further altered by BMT. This stresses the need for longitudinal respiratory monitoring and follow up to detect such dysfunctions and to insure an optimal treatment program for these children.