We report a case of severe hypoglycemia and hepatic masses suspected to be an insulin-like growth factor-II (IGF-II)-producing hepatocellular carcinoma. A 62-year-old man presented with mental disorder in the night and early morning associated with extremely low blood sugar levels (less than 21 mg/dl). Computerized axial tomography and ultrasonography revealed a massive tumor in the right lobe of the liver with multiple secondary nodules, and a tumor thrombus in the portal vein. At autopsy 107 days after admission, the liver weighed 3070 g, histologically showing an Edmondson type II tumor with liver cirrhosis. IGF-II in plasma (899 ng/ml) and tumor tissue (2.4 micrograms/g) was higher than that in normal plasma (374-804 ng/ml) and non-tumor liver tissue (0.2 micrograms/ml), while IGF-I (14 ng/ml) was significantly reduced. IGF-II, probably produced by the liver tumor, appeared to be involved in the mechanism of hypoglycemia.