Hepatocyte paraffin 1 (Hep Par 1) is a monoclonal antibody developed from hepatic tissue from a failed liver allograft. Several studies have shown that Hep Par 1 is a useful marker to differentiate hepatocellular carcinoma (HCC) from other types of adenocarcinoma metastatic to the liver. The aim of our study was the systematic investigation of the epidemiology of Hep Par 1 expression in 3,940 tissue samples using the tissue microarray technique. Strong Hep Par 1 expression was found most frequently in 35 (73%) of 48 HCCs. In nonhepatic tumors, strong Hep Par 1 expression was detected in adenocarcinoma of the lung (2/50), gallbladder (3/31), pancreas (2/48), stomach (3/74), small intestine (1/11), adenoma of the colon with high-grade dysplasia (1/49), adrenal gland carcinoma (1/6), paraganglioma (1/9), and malignant melanoma (2/48). Our data suggest that Hep Par 1 is a highly specific marker for HCC, although several nonhepatic tumors occasionally can show some Hep Par 1 positivity.