From May 1980 through July 1986, 26 patients with severe aplastic anemia, sensitized with multiple transfusions of blood products, were treated on either of two immunosuppressive regimens in preparation for bone marrow transplantation from a matched donor. There were 10 patients treated with total body irradiation (TBI), 200 cGy/fraction X 4 daily fractions (800 cGy total dose), followed by cyclophosphamide, 60 mg/kg/d X 2 d. An additional 16 patients were treated with total lymphoid irradiation (TLI) [or, if they were infants, a modified TLI or thoracoabdominal irradiation (TAI)], 100 cGy/fraction, 3 fractions/d X 2 d (600 cGy total dose), followed by cyclophosphamide, 40 mg/kg/d X 4 d. The extent of immunosuppression was similar in both groups as measured by peripheral blood lymphocyte depression at the completion of the course of irradiation (5% of initial concentration for TBI and 24% for TLI), neutrophil engraftment (10/10 for TBI and 15/16 for TLI), and time to neutrophil engraftment (median of 22 d for TBI and 17 d for TLI). Marrow and peripheral blood cytogenetic analysis for assessment of percent donor cells was also compared in those patients in whom it was available. 2/2 patients studied with TBI had 100% donor cells, whereas 6/11 with TLI had 100% donor cells. Of the five who did not, three were stable mixed chimeras with greater than or equal to 70% donor cells, one became a mixed chimera with about 50% donor cells, but became aplastic again after Cyclosporine A cessation 5 mo post-transplant, and the fifth reverted to all host cells by d. 18 post-transplant. Overall actuarial survival at 2 years was 56% in the TLI group compared with 30% in the TBI group although this was not statistically significant. No survival decrement has been seen after 2 years in either group. There was less long-term morbidity in the TLI group compared with TBI although the numbers of surviving patients are small. With no difference in engraftment or survival, it is suggested that, for sensitized severe aplastic anemia patients, who are to receive a non-T cell-depleted marrow from a matched donor, prudent cytoreduction should include a fractionated, moderate dose irradiation regimen with maximum organ sparing, that is either TLI or TAI.