Traditional severity criteria of sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) were determined retrospectively but found inappropriate for therapeutic decisions. Data of 203 patients with SOS/VOD were collected according to the modified Seattle diagnostic criteria and were analyzed for validation of the revised severity criteria recently proposed from European Society for Blood and Marrow Transplantation (EBMT). According to the traditional severity criteria, none of the patients were mild grade, while 63.1% were moderate and 36.9% were severe grade. However, according to the revised EBMT criteria, the majority of patients (63.1%) were very severe, 18.2% were severe, 12.8% were moderate, and 5.9% were mild grade. The 100-day overall survival (OS) of mild, moderate, severe and very severe groups was 83.3, 84.3, 94.6, and 58.6%, respectively. Very severe SOS/VOD showed a significantly lower OS than the others (58.6 vs. 89.3%, p < 0.0001). The 100-day transplantation-related mortality was 25.2% in the entire cohort; 8.3% in mild, 8.0% in moderate, 2.7% in severe, and 36.7% in very severe SOS/VOD (p < 0.0001). The very severe grade newly classified by the revised EBMT criteria accounted for the majority of SOS/VOD associated with worse 100-day OS. Therefore, intervention should be applied at the latest for moderate to severe SOS/VOD before deterioration.