The best antithymocyte globulin preparation for first-line immune suppression in patients with severe aplastic anemia is still not clear. The aim of this study was to compare hematological response and overall survival in patients submitted to horse or rabbit antithymocyte globulin as first-line treatment for severe aplastic anemia. We retrospectively compared 71 consecutive patients with severe aplastic anemia, classified according to the antithymocyte globulin preparation. Analyses included variables related to patients and to immune suppression. Forty two patients (59.1%) received horse and 29 (40.9%) rabbit antithymocyte globulin. Response rates were higher at 6 months in patients submitted to horse in comparison to rabbit antithymocyte globulin (59.5% versus 34.5% respectively, p = 0.05). Median time to response was similar between the two groups (99 versus 88.5 days, respectively, for horse and rabbit antithymocyte globulin; p = 0.98). Overall survival at 2 years was significantly higher in patients submitted to horse in comparison to rabbit antithymocyte globulin (78.4% versus 55.4%, p = 0.03). Post-treatment response was strongly associated with survival at 2 years (97% in responders versus 41.2% in non-responders, p < 0.001). Use of rabbit antithymocyte globulin was an independent predictor of death (odds ratio 2.5; 95% confidence interval 1.03-6.04; p = 0.04). Rabbit antithymocyte globulin was associated with a significant and prolonged lymphopenia in comparison with horse antithymocyte globulin. Our data suggest the superiority of horse over rabbit antithymocyte globulin as first-line treatment for severe aplastic anemia, both regarding hematological response and survival.