A 65-year-old man with chronic hepatitis C showed a markedly elevated serum alpha-fetoprotein concentration. Computed tomography revealed a huge tumor occupying the entire right hepatic lobe. Three months later, the tumor regressed spontaneously from 12 cm to 7 cm in diameter without any medical treatment. A right hepatic lobectomy was performed 4 months after the initial diagnosis. The main tumor, located in the posterior inferior segment, was completely necrotic, and had a thick fibrous capsule. Many inflammatory cells had also infiltrated into the tumor. Only a small portion of a tumor thrombus in the portal vein and one of three intrahepatic metastases contained viable cancer cells. The tumor was found to be poorly differentiated hepatocellular carcinoma. Tumor regression may have been caused by a disturbance in hepatic circulation associated with the portal vein thrombus.