Hematopoietic cell transplantation (HCT) from an HLA-A, HLA-B, HLA-C, HLA-DRB1, and HLA-DQB1 allele–matched unrelated donor is a well-recognized life-saving treatment modality for patients with hematologic disorders. The morbidity and mortality from clinically significant acute graft-versus-host disease (aGVHD) remains a limitation. The extent to which transplantation outcome may be improved with donor matching for HLA-DP is not well defined. The risks of aGVHD, relapse, and mortality associated with HLA-DPB1 allele mismatching were determined in 5929 patients who received a myeloablative HCT from an HLA-A–, HLA-B–, HLA-C–, HLA-DRB1–, and HLA-DQB1–matched or –mismatched donor. There was a statistically significantly higher risk of both grades 2 to 4 aGVHD (odds ratio OR = 1.33; P < .001) and grades 3 to 4 aGVHD (OR = 1.26; P < .001) after HCT from an HLA-DPB1–mismatched donor compared with a matched donor. The increased risk of aGVHD was accompanied by a statistically significantly decrease in disease relapse (hazard ratio HR = 0.82; P = .01). HLA-DPB1 functions as a classical transplantation antigen. The increased risk of GVHD associated with HLA-DPB1 mismatching is accompanied by a lower risk of relapse. Knowledge of the DPB1 matching status prior to transplantation will aid in more precise risk stratification for the individual patient.