Hepatic vein stenosis is a vascular complication that can lead to graft loss after liver transplantation. Although ascites frequently occurs as a symptom of hepatic vein stenosis, the development of severe hypogammaglobulinemia associated with hepatic vein stenosis has not been reported in the literature. An 8-year-old boy underwent living-related liver transplantation (LRLT) because of Wilson disease with chronic hepatic failure. Because de novo autoimmune hepatitis was diagnosed 1 year after LRLT, azathioprine, and prednisolone were added to the baseline immunosuppression of tacrolimus. The patient developed ascites with severe hypogammaglobulinemia (immunoglobulin G [IgG], 288 mg/dL) 2 years after LRLT. Ultrasonography and angiography disclosed stenosis of the hepatic vein. The ascites completely resolved after percutaneous balloon angioplasty. Despite serum IgG trough levels of >500 mg/dL maintained by the addition of immunoglobulin, cytomegalovirus reactivation and sepsis occurred. Serum IgG levels should be monitored to prevent opportunistic infections when hepatic vein stenosis is diagnosed after LRLT.