From July 1985 to December 1989, 72 evaluable patients aged between 6 and 51 (median age 27 years) suffering from haematological malignancies received an allogeneic bone marrow transplant (BMT) depleted of T-lymphocytes to reduce the risk of graft-versus-host-disease (GvHD); 57 were matched and 15 mismatched. Three different conditioning regimens were used in an effort to enhance cytoreduction without increase extramedullary toxicity. Mismatched patients were treated with more immunosuppressive regimens. Total body irradiation (TBI) was given in three doses per day, 5 h apart, over 4 days for a total of 12 fractions. The dose to the lungs was 14.4, 15.6 and 9 Gy according to the conditioning regimen. The incidence of interstitial pneumonia (IP) was 12.3% in matched and 46.7% in mismatched patients. Our results seem to indicate that lung toxicity is correlated with the intensity of the conditioning regimen, the stage of disease and, in mismatched patients, with the degree of human leucocyte antigen (HLA) disparity and the poor post-BMT reconstitution, rather than the radiotherapy dose delivered to the lungs. On the contrary, the hyperfractionated scheme adopted, the absence of GvHD and, perhaps, the post-TBI administration of cyclophosphamide all seem to have contributed to the low incidence of IP in our matched patients.