Sinusoidal obstruction syndrome (SOS), also known as hepatic veno-occlusive disease (VOD), is a life-threatening complication of hematopoietic stem cell transplantation. In severe cases, SOS/VOD progresses to multiple organ failure with a mortality rate higher than 80%. Early diagnosis and treatment based on severity assessment improve the prognosis of severe SOS/VOD, but conventional diagnostic criteria may be insufficient for an early diagnosis. We herein report a case of severe late-onset SOS/VOD that was histologically proven before clinical findings become evident. Although defibrotide therapy was started before the clinical diagnostic criteria were met, the disease progressed to multiple organ failure. This case suggests that some patients require earlier treatment before the appearance of clinical findings such as ascites and weight gain or hyperbilirubinemia, and supports the need to establish methods for predicting the onset and severity of SOS/VOD.