We investigated the clinical significance of serum and intrahepatic KL-6/MUC1 (KL-6) in patients with hepatitis C virus (HCV) antibody-positive hepatocellular carcinoma (HCC). The subjects included 76 patients diagnosed with anti-HCV positive HCC, 69 with, and 51 without, liver cirrhosis (LC). Frozen serum samples were obtained from each subject to determine the serum KL-6 levels using an enzyme-linked immunosorbent assay. Expression of KL-6 antigen in the liver was also investigated using immunoperoxidase staining. The mean serum KL-6 level in patients with HCC was [Formula: see text] U/ml (319U/ml for HCC with LC, 342.8U/ml for HCC without LC). Serum KL-6 levels in patients with HCC with LC and HCC without LC did not differ. Serum KL-6 levels were elevated with increases in the size of spaces occupied by tumors in the liver. Among patients with HCC, there was no correlation between serum KL-6 levels and alpha-fetoprotein (AFP) levels and protein induced by vitamin K absence or antagonist-II (PIVKA-II) levels. However, some patients with low levels of AFP and PIVKA-II possessed high levels of KL-6. Furthermore, serum KL-6 levels decreased after therapy for HCC nodules. Immunohistochemical staining showed KL-6 antigen was detected within the cell membrane and in the cytoplasm of cancer cells. KL-6 antigen was localized on the membrane and the endoplasmic reticulum of cancer cells in the cancerous foci by electron microscopy. Our results suggest that serum KL-6 levels represent a serological marker of HCC development, because KL-6 expression was localized to the cancer cells in HCC nodules.