Interstitial pneumonia is one of the major causes of morbidity and mortality after bone-marrow transplantation. We here report a series of 58 patients suffering from hematological malignancies who received HLA-matched T-lymphocyte depleted bone-marrow transplants between July 1985 and January 1990. Interstitial pneumonia occurred in 7/58 patients (12%) and was fatal in six. Three different pre-bone-marrow transplantation conditioning regimens were employed. Total body irradiation was delivered according to a hyperfractionated scheme of 12 fractions given three per day 5 hr apart for 4 days. Twenty-three patients received 36 mg/Kg procarbazine, 1275 UL/Kg antithymocite globulin, 14.4 Gy hyperfractionated total body irradiation and 120 mg/Kg cyclophosphamide. Only one patient developed interstitial pneumonia, but two rejected the graft and 10 relapsed. As a consequence, the total hyperfractionated scheme was increased to 15,6 Gy, cyclophosphamide to 200 mg/Kg, antithymocite globulin to 3400 UL/Kg and procarbazine eliminated. There were three cases of interstitial pneumonia, no rejection and four relapses in the 17 patients who received this regimen. In the last 18 patients hyperfractionated total body irradiation was reduced to 15 Gy, cyclophosphamide to 100 mg/Kg, and 10 mg/Kg of the myeloablative agent thiothepa added to enhance the cytoreductive effect without significantly increasing extramedullary toxicity. Three cases of interstitial pneumonia, one relapse but no rejection were recorded. Our results demonstrate that the absence of graft-versus-host disease due to T-cell depletion, and radio-chemotherapy doses and schedules used for the conditioning regimen each contributed to reducing the risk of interstitial pneumonitis. Hyperfractionated total body irradiation therefore, seems to play an important role in lowering the incidence of this complication.