Busulfan and the metabolites of cyclophosphamide are conjugated with glutathione and catabolized by enzymes of the cytosolic glutathione S-transferases family. There are clearly linked single nucleotide polymorphisms in the promoter region of the glutathione S-transferase A1 gene (i.e., GSTA1*A, −567T, −69C and −52G; GSTA1*B, −567G, −69T and −52A). We assessed whether the clinical outcomes, including acute graft-vs.-host disease, of 61 patients with hematological malignancies, following HLA-matched sibling allogeneic stem cell transplantation using busulfan/cyclophosphamide conditioning are altered by glutathione S-transferase A1 genotypes. Globally, grade II–IV acute graft-vs.-host disease developed in 13 patients (21%). Grade II–IV acute graft-vs.-host disease developed in 15.2% of 46 patients with GSTA1*A/*A diplotype and in 40.0% of 15 patients with GSTA1*A/*B or GSTA1*B/*B diplotype (p = 0.04). Moreover, this relationship between GSTA1*A/*A diplotypes and lower incidence of acute graft-vs.-host disease was independent of the age, gender, stem cell source, and disease status. The incidences of acute skin graft-vs.-host disease were 7% (3/46) in patients with GSTA1*A/*A and 27% (4/15) in patients without GSTA1*A/*A (p = 0.009, univariate; p = 0.01, multivariate). Acute hepatic graft-vs.-host disease developed in 6 (13%) of 46 patients with the GSTA1*A/*A diplotype and in 4 (27%) of 15 patients without this diplotype (p = 0.09, univariate; p = 0.12, multivariate). Ten patients (16%) developed hepatic veno-occlusive disease. No significant difference was found in the incidence of hepatic veno-occlusive disease between patients with and without the GSTA1*A/*A diplotype (19.6% vs. 6.7%; p = 0.24). We conclude that the GSTA1*A/*A diplotype is an independent protective factor against acute graft-vs.-host disease, especially for skin graft-vs.-host disease, and probably for hepatic graft-vs.-host disease, in patients using busulfan/cyclophosphamide conditioning. The identification of glutathione S-transferase A1 genotypes prior to allogeneic stem cell transplantation could allow conditioning regimens and graft-vs.-host disease prophylaxis to be modified to improve outcome.