We present a case of rapid onset of glycogen storage hepatomegaly, caused by a massive dose of long-acting insulin and large doses of glucose, in a type-2 diabetic patient. A 41-year-old man was admitted to our hospital because of hypoglycemia and unconsciousness following subcutaneous administration of 180 units of insulin glargine in a suicide attempt. Despite continuous hypercaloric infusion with additional intravenous glucose injections, hypoglycemia persisted for 36 hours. Although the hepatic function was normal and no hepatomegaly was detected on admission, the liver function tests became abnormal and hepatomegaly was detected on hospitalization day 3. Plain abdominal computed tomography (CT) scanning confirmed liver enlargement, with hepatic CT attenuation markedly elevated at 83.7 HU. Liver biopsy revealed hepatocytic glycogen deposition with edematous degeneration. Based on these findings, the diagnosis was made as rapid onset glycogen storage hepatomegaly caused by administration of a massive dose of long-acting insulin and supplementation with large doses of glucose. With improved glycemic control, the liver function improved, the CT findings of hepatomegaly improved, and the hepatic CT attenuation decreased. Repeat liver biopsy also confirmed almost complete disappearance of glycogen deposits. When hepatic dysfunction or hepatomegaly is detected during treatment with insulin, the possibility of hepatic glycogen deposition should be considered. CT scanning and liver biopsy were useful in diagnosing this case.