Acute toxicity, delayed complications and the incidence of interstitial pneumonitis (IP) after total body irradiation (TBI) are reviewed. Between July 1979 and March 1986 118 patients with hematological malignancies underwent bone marrow transplantation (BMT): all were conditioned with cyclophosphamide and TBI. 106 patients received single dose irradiation (18 with lung-shielding) and 12 had fractionated TBI. Except for mucositis all other symptoms of acute toxicity such as fever, vomiting, parotitis, headache and abdominal pain were usually of short duration. Fractionated TBI did not produce less acute side effects than single dose irradiation. Irradiation, and particularly the maximum lung dose, plays an important role in the multifactorial pathogenesis of IP. Delayed complications may appear several months or years after TBI. The majority of men suffer definitive sterility caused by azoospermia. In women primary ovarian failure and permanent sterility is a common sequela of BMT with TBI. The risk of developing cataracts after single dose TBI is about 80%, and surgical repair was necessary in 43% of these cases. Secondary tumors after TBI have been reported. So far we have observed no secondary malignancies in our patients after BMT. Rampant dental decay can be avoided by careful dental prophylaxis.