AIM: To analyze the influence factors and formation of extrahepatic collateral arteries (ECAs) in unresectable hepatocellular carcinoma (HCC) with or without chemoembolization.METHODS: Detailed histories of 35 patients with 39 ECAs of HCC and images including computerized tomography scan, digital subtraction angiography were reviewed carefully to identify ECAs of HCC, ECAs arising from, and anatomic location of tumors in liver. Tumor sizes were measured, and relations of EC As with times of chemoembolization, tumor size, and the anatomic tumor location were analyzed. Complications were observed after chemoembolization through ECAs of HCC with different techniques. RESULTS: Influence factors of formation of ECAs of HCC included the times of repeated chemoembolization, thelocation of tumors in liver, the tumor size and the types of chemoembolization. ECAs in HCC appeared after 3-4 times of chemoembolization (17.9%), but a higher frequency of ECAs occurred after 5-6 times of chemoembolization (56.4%). ECAs presented easily in peripheral areas (71.8%) of liver abutting to the anterior, posterior abdominal walls, the top right of diaphragm and right kidney.ECAs also occurred easily after complete obstruction of the trunk arteries supplying HCCs or the branches of proper hepatic arteries. Extrahepatic collaterals of HCC originated from right internal thoracic (mammary) artery (RITA, 5.1%), right intercostal artery (RICA, 7.7%), left gastric artery (LGA, 12.8%), right inferior phrenic artery (RIPA, 38.5%),omental artery (OTA, 2.6%), superior mesenteric artery (SMA, 23.1%), and right adrenal and renal capsule artery (RARCA, 10.3%), respectively. The complications after chemoembolization attributed to no super selective cathet erization.CONCLUSION: The formation of ECAs in unresectable HCCis obviously correlated with multiple chemoembolization,tumor size, types of chemoembolization, anatomic locationof tumors. Extrahepatic collaterals in HCC are corresponding to the tumor locations in liver.