We first describe a case of generalized intraperitoneal seeding of hepatocellular carcinoma (HCC) after microwave coagulation therapy (MCT). A 61 year-old man underwent operative MCT for an exophytic HCC, 60 mm in diameter, in segment IV of his cirrhotic liver. Despite successful tumor ablation, the serum alpha-fetoprotein levels continuously rose after MCT. Five months later, radiographic examinations delineated several perihepatic masses with hypervascularity, and the patient presented with constipation. At the second laparotomy, there were numerous small peritoneal metastases involving the entire peritoneal cavity and slightly bloody ascites. An omental mass, 50 mm in diameter, involved the transverse colon. Most of these intraabdominal masses were removed together with the involved colon. Histologically, the initial tumor was a moderately differentiated HCC, and the peritoneal masses were poorly differentiated HCCs. The patient died of rapid tumor progression and bleeding 2 months later. In conclusion, we should be aware of the possible occurrence of peritoneal seeding after MCT for HCC. Every effort should be made to prevent this serious complication, particularly in cases of superficial, large, and less differentiated HCCs.