IL-17 has an important role in the host defense against extracellular pathogens and the pathophysiology of autoimmune diseases. This study retrospectively examined the impact of a single-nucleotide polymorphism (rs2275913, G197A) in the IL-17 gene of a total 510 recipients with hematologic malignancies and their unrelated donors on the clinical outcomes in HLA-matched myeloablative (discovery study) and nonmyeloablative (validation study) BMT through the Japan Marrow Donor Program (JMDP). In the discovery study, the presence of a 197A genotype in the recipient resulted in a higher incidence of grades II-IV acute GVHD (hazard ratio (HR), 1.87; 95% confidence interval (CI), 1.23-2.85; P=0.004). The donor IL-17A genotype did not significantly influence the transplant outcomes. The validation study showed a trend toward an association of the recipient 197A genotype with an increased risk of grades III-IV acute GVHD (HR, 5.84; 95% CI, 0.75-45.72; P=0.09), as well as a significantly increased risk for chronic GVHD (HR, 3.86; 95% CI, 1.29-11.59; P=0.02). These results suggest an association of the 197A genotype in the recipient side with the development of acute GVHD.