Although cytokeratin (CK) phenotyping of metastatic tumors is now routine in many laboratories, the clinical relevance of the procedure has seldom been addressed. We carried out a prospective clinical study of 134 consecutive cases of metastatic adenocarcinoma of the liver diagnosed by needle biopsies stained routinely for CK20 and CK7. The most probable localization of the primary tumor, deduced from this staining pattern, was stated in the original pathology report. The present study compared this assignment with the information available at the time of interpretation of the liver biopsy, to the results of the subsequent clinical investigation, and to the officially reported cause of death as outcome. As expected, the primary tumors were localized in the colon or in the rectum in 85% (34/40) of the CK20+/CK7− metastases. The definite diagnosis remained metastatic colorectal carcinoma in 83% (15/18) of the cases with diagnosed colorectal cancer before the liver biopsy. In the cases without a known primary tumor when the liver biopsy was interpreted, primary colorectal localization was accurately predicted in 86% (19/22) of the patients. Compared to the outcome, 77% (36/47) of the CK20+/CK7+ metastases had the expected pancreaticobiliary primary localization in 83% (30/36) without any primary tumor being known at the time of interpretation of the liver biopsy. In contrast, the majority of CK20− metastatic carcinomas had an unexpected primary localization, 50% (16/32) in the CK20−/CK7+ and 60% (9/15) in the CK20−/CK7− subgroup. In addition, the origin of the liver metastasis remained unknown in 37% (12/32) of CK20−/CK7+ cases. Thus, the CK20+/CK7− phenotype indicates a colorectal origin of the liver metastasis with considerable accuracy and independently of the available clinical information. The same is true for CK20+/CK7+ metastases, which indicate primary tumor localization in the pancreas or in the biliary tree. The results in the CK20− subgroups of the liver metastases are disappointing and cannot substantially help the clinical investigation.