Despite the success of human leukocyte antigen (HLA) typing in allogeneic stem cell transplantation (SCT) it is rare to find an unrelated donor that is perfectly matched, making identification of "permissive" mismatches of paramount importance. Here, we describe novel associations between donor T-cell cytokine production during donor-antipatient mixed lymphocyte reactions (MLRs) and acute graft-versus-host disease (aGVHD). The data reveal positive correlations between both Th1-type and Th2-type cytokine production and GVHD and the assay established could potentially represent a useful tool for identification of permissible unrelated SCT donors. Associations between interleukin 13 (IL-13) levels and aGVHD were by far the strongest predictor of a GVHD (P =.0002). All patients suffering severe (grade III) aGVHD following SCT had donors who produced very high pretransplantation IL-13 responses, while those developing little or no aGVHD (grades 0-I) produced no IL-13 at all. IL-13 levels were independent of all other cytokines measured as well as cytotoxic T-lymphocyte precursor (CTLp) frequencies. The cytokines IL-5, interferon gamma (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha) also predicted development of aGVHD (P <.05 for all 3), appearing to be coproduced in the assay and correlating with estimated CTLp frequencies. The data challenge the notion that aGVHD is purely a Th1-type cytokine-driven response, high-lighting a novel and highly significant link between the Th2-type cytokine IL-13 and aGVHD.