Oral squamous cell carcinoma (OSCC) cells can invade adjacent tissues and the vascular system at an early stage of tumor progression. In the present study, we have attempted to detect circulating cancer cells. We used human OSCC cells expressing green fluorescent protein (GFP) in an orthotopic nude mouse model and were able to detect GFP-expressing cells using a fluorescence activated cell sorter and fluorescence microscopy. We also detected the expression of GFP, squamous cell carcinoma antigen (SCCA), or epidermal growth factor receptor (EGFR) mRNA in the blood of tumor-bearing mice by conventional reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time quantitative RT-PCR (TaqMan RT-PCR). TaqMan RT-PCR was the more sensitive method to detect the circulating cancer cells. Furthermore, we examined the expression of SCCA and EGFR mRNA in the peripheral blood of patients with OSCC by conventional RT-PCR and TaqMan RT-PCR. We detected SCCA and EGFR mRNA in few cases by conventional RT-PCR, whereas TaqMan RT-PCR showed their expression in about 50% of cases. However, there was no correlation between detection of circulating viable cancer cells and metastasis to lymph nodes and distant organs. These results suggest that TaqMan RT-PCR is a very useful method and both SCCA and EGFR mRNA may be available as a marker for detection of circulating cancer cells.