Late renal dysfunction following allogeneic bone marrow transplantation has been described by a number of centers including our own. Total body irradiation appears to play a major causative role in the development of this syndrome. In an effort to decrease the incidence of this renal toxicity we have added customized partial transmission renal blocking to our total body irradiation regimen. This partial renal shielding decreases the total dose to the kidneys from 14 Gy to 12 Gy. This report compares 71 adult patients who have received total body irradiation associated with bone marrow transplantation using renal shielding with 72 adult patients who were treated without the shielding; all of the patients have survived a minimum of 100 days post-BMT. Eighteen months following BMT, 26% of patients who did not have renal shielding have developed late renal dysfunction compared to only 6% of patients with renal shielding (p less than .05). Median follow-up in the nonblocked patients is 536 days post-transplant versus 341 days for the blocked patients. This added renal blocking has not adversely affected engraftment rates or relapse rates to date. Customized renal shielding as part of 14 gray total body irradiation in preparation for bone marrow transplantation appears to have decreased the incidence of late renal dysfunction in this group of adult patients and should be considered for all patients undergoing total body irradiation in conjunction with bone marrow transplantation.