Second allogeneic hematopoietic stem cell transplantation is a curative treatment option for patients with hematologic malignancies. However, it is unclear whether HLA discrepancy between graft and first donor has an impact on the outcome of second transplantation. We retrospectively analyzed 646 patients receiving second transplantation after an initial HLA mismatched transplantation. With regard to graft-versus-host, the one-allele mismatch (1 mismatch) group (SHR, 1.88; 95% CI: 0.79-4.45; P=0.163) and more than one-allele mismatch group (>= 2 mismatch) (SHR, 1.84; 95% CI, 0.75-4.51; P=0.182) had higher risks of grade III-IV acute graft-versus-host disease (GvHD) compared to the HLA-matched (0 mismatch) group. In contrast, no difference in risk of acute GvHD was found among the 0, 1, and >= 2 mismatch group with respect to graft-versus-first donor. With regard to graft-versus-host, the >= 2 mismatch group showed a significantly higher risk of treatment-related mortality (SHR, 1.90; 95% CI, 1.04-3.50; P=0.038) compared to the 0 mismatch group, while the risk of relapse was slightly lower in the >= 2 mismatch group (SHR, 068; 95% CI, 0.44-1.06; P=0.086). In contrast, with regard to