Antithymocyte globulin (ATG) is commonly used for graft-vs.-host disease (GVHD) prophylaxis in unrelated donor allogeneic transplantation (Allo-HSCT). However, its use is still controversial in matched sibling donor (MSD) Allo-HSCT, notably after reduced intensity conditioning (RIC). ATG dose may influence the outcome, explaining in part the discordant conclusions in MSD Allo-HSCT. We, therefore, analyzed the impact of ATG doses in patients with acute myeloid leukemia in first complete remission undergoing RIC Allo-HSCT from a MSD. We analyzed 234 patients from the EBMT registry and compared outcome according to given ATG dose (high dose: ≥ 6 mg/kg, n = 39 or low dose: < 6 mg/kg, n = 195). No difference was found in the cumulative incidence of acute (grade 2-4: high dose vs. low dose: 21% vs. 13%, p = 0.334; adjusted hazard ratio (HR): 1.20, p = 0.712) and chronic GVHD (extensive: high dose vs. low dose: 19% vs. 18%, p = 0.897; adjusted HR: 1.01, p = 0.980). In contrast, high dose of ATG significantly increased the incidence of relapse (52% vs. 26%, p = 0.011; adjusted HR: 1.31, p = 0.001) leading to impaired outcome (HR progression-free survival (PFS): 1.23, p = 0.002; HR overall survival (OS): 1.17, p = 0.029; HR GVHD and relapse-free survival (GRFS): 1.20, p = 0.005). We conclude that an ATG dose <6 mg/kg is sufficient for GVHD prophylaxis, while higher doses impair disease control and outcome. © 2017 The Author(s) 2017, under exclusive licence to Macmillan Publishers Limited, part of Springer Nature.