Total body irradiation was used in 22 patients as part of their conditionning regimen for bone marrow transplantation. Nine patients with acute leukemia received 1 000 cGy TBI in addition with chemotherapy. None of them survived and the main cause of death was interstitial pneumonitis (50%). 4 patients received 1 000 cGy with a lung shielding of 500 cGy. Two patients with acute leukemia died of leukemia and sepsis, two patients had aplastic anemia, one is surviving, the other died of severe GVHD and infectious complications. Nine patients with severe aplastic anemia strongly immunized by previous blood transfusions received 800 cGy TBI with a lung shielding of 400 cGy. No rejection was observed and 7 patients (63%) are currently alive. One patient died of interstitial pneumonitis probably related to CMV infection, one of subacute necrotizing hepatitis, two of severe acute GVHD. It is concluded from this study that TBI remains the best immunosuppressive conditioning regimen even in strongly immunized patients. It may be a contributing factor of the incidence and severity of interstitial pneumonitis. A reduction of the dose ot the lung to 400-500 cGy seems to decrease the severity of this complication.