We performed immunohistochemical studies on 90 surgically resected liver tumors, including 30 tumors each from hepatocellular carcinoma (HCC), cholangiocarcinoma (CC), and metastatic colorectal adenocarcinoma (MCA), using monoclonal antibodies against cytokeratin (CK) 7, CK 19, and CK 20 to examine the differences in the CK expressions in primary and metastatic carcinomas of the liver. We also investigated the usefulness of such expression in the differential diagnosis in addition to existing markers such as alpha-fetoprotein, carcinoembryonic antigen, and carbohydrate antigen 19-9. For CK 7, all except for one (97%) of the CCs were diffusely positive, whereas only two (7%) HCCs and one (3%) MCAs were diffusely positive. For CK 19, 23 (77%) CCs and 19 (64%) MCAs were diffusely positive, whereas no HCCs were positive. For CK 20, 22 (74%) MCAs were diffusely positive, whereas no HCC and three (10%) CCs were diffusely positive. The findings concerning the expression of immunohistochemical CK are therefore considered to be useful in addition to the diagnostic criteria when making a differential diagnosis of primary and metastatic carcinomas of the liver.