Objectives: Hepatocellular carcinoma (HCC) is one of the common cancers worldwide. Accurate diagnosis of tumor progression is critical for the appropriate management of HCC. Here, we established a sensitive assay to detect and quantify tumor-derived DNA in the serum of HCC patients. Methods: Aberrant methylation of the APC gene was quantified in 23 HCC patients and 8 healthy volunteers using 100 mu l of serum. For sensitive detection and accurate quantification of tumor DNA, we combined seminested polymerase chain reaction (PCR) with TaqMan PCR, which could amplify the APC gene regardless of the methylation status and detect the methylated and unmethylated sequences separately. The ratio of methylated to unmethylated sequences was quantified. Results: The methylated APC gene was detected in all HCC patients examined, but no healthy volunteers showed amplification of methylated sequences in serum. HCC patients with portal vein thrombosis showed a significantly higher methylated to unmethylated APC gene ratio in serum than those without portal vein thrombosis (p = 0.0029). Conclusions: Considering the strong association between the ratio of the methylated to unmethylated APC sequences in serum and the presence of portal vein thrombosis, methylation status of APC sequences could be a promising marker for improving HCC management. Copyright (C) 2013 S. Karger AG, Basel