Primary neoplasms of the liver are composed of cells that resemble the normal constituent cells of the liver. Hepatocellular carcinoma, in which the tumor cells resemble hepatocytes, is the most frequent primary liver tumor, and is highly associated with chronic viral hepatitis and cirrhosis of any cause. Benign tumors, such as hepatocellular adenoma in a noncirrhotic liver or a large, dysplastic nodule in a cirrhotic liver, must be distinguished from well-differentiated hepatocellular carcinoma. Cholangiocarcinoma, a primary adenocarcinoma that arises from a bile duct, is second in frequency. It is associated with inflammatory disorders and malformations of the ducts, but most cases are of unknown etiology. Cholangiocarcinoma resembles adenocarcinomas arising in other tissues, so a definitive diagnosis relies on the exclusion of an extrahepatic primary and distinction from benign biliary lesions.