Allogeneic stem cell transplantation (ASCT) has improved leukemia-free survival (LFS) in many but not all patients with acute leukemia. This is an eight-year follow-up to our previous study showing a survival advantage to patients with an increased γδ T cells following ASCT. γδ T cell levels were collected prospectively in 153 patients (acute lymphoblastic leukemia (ALL) n= 77; acute myelogenous leukemia (AML) n= 76) undergoing partially mismatched related donor ASCT. Median age was 22 years (1–59), and 62% of the patients were in relapse at transplant. Patient–donor human leukocyte antigen (HLA) disparity of three antigens was 37% in the graft-versus-host disease (GvHD) and 29% in the rejection directions. All patients received a partially T cell-depleted graft using T10B9 (n= 46) or OKT3 (n= 107). Five years LFS and overall survival (OS) of patients with increased γδ compared to those with normal/decreased numbers were 54.4 vs 19.1%; P< 0.0003, and 70.8 vs 19.6% P< 0.0001, respectively, with no difference in GvHD (P= 0.96). In a Cox multivariate analysis, normal/decreased γδ (hazard ratio (HR) 4.26, P= 0.0002) and sex mismatch (HR 1.45 P= 0.049) were associated with inferior LFS. In conclusion, γδ T cells may facilitate a graft-versus-leukemia (GvL) effect, without causing GvHD. Further evaluations of this effect may lead to specific immunotherapy for patients with refractory leukemia.