The Medical College of Wisconsin implemented a major bone marrow transplant (BMT) program in July 1985. The type of transplants to be focused on were allogeneic T-lymphocyte deplete. Total body irradiation (TBI) was initially patterned after the Memorial method. Patients received total body irradiation in a sitting position at a dose rate of 20-25 cGy/minute with 50% attenuation lung blocks used both anterior/posterior and posterior/anterior. Electron boosting was utilized for the ribs beneath the lung blocks. Occasionally, lower extremity boosting was required because of the sitting position. A dose of 14 Gy was chosen since T-lymphocyte deplete bone marrow transplant data suggest the need for higher total doses to consistently obtain engraftment. This dose was given in 3 equal daily fractions over 3 days following conditioning chemotherapy. Six of 11 patients treated in this manner developed lethal pulmonary events. In response to the pulmonary toxicity, partial lung shielding was increased to 60% attenuation. In the next 107 patients receiving this program of total body irradiation there was a reduced incidence of fatal pulmonary events (10 cases of fatal idiopathic interstitial pneumonitis and 12 cases of fatal pulmonary infections) after a median follow-up of 9 months. This was an obvious improvement over the initial group. A significant level of hepato-renal toxicity was also observed with 14 Gy total body irradiation when no liver or kidney blocking was used. Of the first 20 patients treated, three cases of fatal veno-occlusive disease resulted. Subsequently, a 10% attenuation right sided liver block was added. Five of 98 patients treated with this block have developed fatal hepatic dysfunction, (median follow-up of 7.2 months). This incidence is not statistically different from the initial group but favors the use of the liver block. Some renal toxicity was also detected with the earlier regimen, especially in pediatric patients. Partial kidney blocking has been implemented to minimize this toxicity. Our current dose rate has been reduced to 8 cGy/minute in a further attempt to reduce organ toxicity. To date, this selective blocking has not adversely affected the excellent rate (96%) of first time engraftments.