Real-time reverse transcription-polymerase chain reaction (RT-PCR) can detect mRNA of carcinoembryonic antigen (CEA) quantitatively. We compared the serum concentration, localization, and mRNA expression of CEA to determine the relationship between these three factors in patients with lung cancer. Tumors from ten patients who underwent surgery were analyzed. The serum concentration of CEA was measured before initiating therapy using a quantitative latex agglutination reaction. Immunohistochemical staining of pathologic specimens of resected tumors was performed to localize CEA. Real-time RT-PCR to detect mRNA of CEA was performed for isolating RNA from a piece of fresh-frozen tumor. Positivity for CEA production was 20% for serum, 60% for immunohistochemistry, and 80% for real-time RT-PCR. Thus, the percentage of mRNA and protein positivity of CEA in lung cancer was much higher than for the serum CEA concentration. No statistically significant correlation between the serum CEA concentration and the amount of mRNA expression was found (p=0.0932). Real-time RT-PCR is useful to quantify specific mRNA expression from a small piece of tissue.