The diagnostic utility of aquaporin (AQP)-1 in liver tumors was tested and compared with other well-established markers. In 30 cholangiocarcinomas (CCs), 20 hepatocellular carcinomas (HCCs), and 10 metastatic colorectal carcinomas (MCCs) of the liver, expression of AQP-1, CD10, cytokeratin (CK) 7, CK20, and polyclonal carcinoembryonic antigen (pCEA) was tested. In addition, staining patterns of CD10 and pCEA were analyzed. To compare the selectivity of AQP-1 and CK7 as possible markers for differentiated cholangiocytes, liver biopsies of cholestatic disease were also analyzed. Aquaporin-1 expression was found in 93% of all CCs compared with 0% of HCC (P < .000001) and with 30% of MCC (P < .01). CD10 was positive in 16.7% of CC compared with 40% of HCC (P < .04) and to 20% of MCC (not significant). Cytokeratin 7 was positive in 90% of CC compared with 10% of HCC (P < .00001) and with 20% of MCC (P < .0001). Cytokeratin 20 was positive in 90% of MCC compared with 16.7% of CC (P < .0001) and with 20% of HCC (P < .00001). Canalicular staining patterns of CD10 and pCEA were observed in HCC (100% and 89.5%, respectively) but typically not in CC (0% and 6.7%, respectively) and never in MCC. In cholestatic disease, AQP-1 was expressed in differentiated epithelial cells of the bile ducts, whereas CK7-positive hepatocytes of Rappaport zone 1 did not show any AQP-1 reactivity. Therefore, AQP-1 seems to be a highly selective marker for differentiated cholangiocytes and can be very helpful in the differential diagnosis of liver tumors.