Acute graft-versus-host disease (AGvHD) is a significant cause of morbidity and mortality in patients receiving a bone marrow transplant from an unrelated donor, and in an effort to reduce this problem, donors are selected for the least possible HLA incompatibility with the recipient. Selection criteria have included minimal incompatibility for the HLA-A, -B, and -DR loci and low reactivity in mixed lymphocyte culture (MLC); however, the value of MLC reactivity for prediction of development of AGvHD has been questioned. We therefore examined the correlation of MLC reactivity with AGvHD in recipients of unrelated bone marrow transplants. Reactivity in the GvH direction was assessed as relative response (RR) of donor lymphocytes to recipient stimulator lymphocytes. In 126 transplanted pairs with technically satisfactory MLC tests, the RR was divided into quartiles (0-1, 2-5, 6-16, and 17-117% RR). HLA-DRB1 incompatibilities were more frequent in the highest quartile (P < 0.001); there were no significant differences among quartiles in donor or recipient age, diagnosis, or frequency of HLA-A or -B incompatibility. Incidence of AGvHD during the first 100 days post-transplant was assessed by Kaplan-Meier analysis. There was no significant difference in incidence of AGvHD among quartiles for the entire group of 126 pairs, for a subset with hematologic malignancy, for a subset selected by a more stringent standard for "technically satisfactory" MLC, or for a subset matched for A, B, and DRB1. The MLC response of donor lymphocytes to recipient stimulator lymphocytes is thus not predictive of development of AGvHD in our patient population receiving unrelated donor bone marrow. Since there was no difference in mortality related to high and low MLC responses, our data also suggest that MLC results are not predictive of survival in this population.