We retrospectively investigated the incidence, risk factors, and outcomes of SOS (sinusoidal obstruction syndrome; previously veno-occlusive disease [VOD]) after allogeneic hematopoietic stem cell transplantation (alloHSCT) in aplastic anemia. Two hundred and sixty patients were included in the analysis. SOS developed in 7.3% (n=19/260) of patients. Classical Cy (200 mg/m(2))-ATG was the most common conditioning regimen (84.2%) in the SOS group. The SOS mortality rate was 4/19 (21.1%). Univariate analyses revealed that Cy 200 mg/m(2) conditioning (p=0.035), classical Cy-ATG conditioning (p=0.007), and horse ATG conditioning (p<0.001) were significant risk factors for developing SOS. Multivariate analysis revealed that only horse ATG conditioning was a poor prognostic factor (HR=3.484; 95% CI 1.226-9.904; p=0.002). Rabbit ATG (HR 12.719; 95% CI 2.332-69.373; p=0.003) and weight gain>10% (HR 35.655; 95% CI 2.208-575.805; p=0.012) were risk factors in the overall SOS group. Both rabbit ATG conditioning and weight gain of more than 10% were associated with poor overall survival with a median of 1.2 months (5Y survival rate, any risk factor vs. none: 74.6% vs. 0.0%; p<0.001; Fig. 2) in the SOS group. In conclusion, SOS is a relatively rare (7.3%) but highly fatal (21.1%) acute complication of alloHSCT in AA, and the horse ATG conditioning regimen was a significant risk factor for developing SOS.