This study aimed to assess the impact of antithymocyte globulin (ATG) on patient outcome in a retrospective series of 91 patients (median age: 12 years) who underwent unrelated single-unit cord blood transplantation (allo-CBT) following a myeloablative conditioning regimen. Cord blood units were HLA-matched (6/6, n=18; 21%), one-Ag mismatched (n=30, 35%) or two-Ag mismatched (n=38; 44%). In this series, the OS, nonrelapse mortality (NRM) and cumulative incidence of relapse were 47±6%, 23±4% and 48±5%, respectively. Among 46 patients who received ATG as part of the conditioning regimen, the incidence of acute and chronic GVHD was lower than that in the group of 45 patients who did not receive ATG (20% vs 43%; P=0.03). However, multivariate statistical analysis revealed that the ATG use was associated with decreased OS and EFS rates and a high incidence of NRM (hazard ratio (HR)=1.99, 95% confidence interval (CI): 1.11-3.59, P=0.02), (HR=1.83, 95% CI: 1.08-3.10, P=0.02) and (HR=2.54, 95% CI: 1.03-6.26, P=0.04), respectively. Therefore, our results do not support the use of ATG as part of a myeloablative-conditioning regimen before single-unit allo-CBT in younger patients with hematological malignancies.