To detect circulating RCC cells, we established a nested RT-PCR system for cadherin-6 mRNA, which is specifically expressed in RCC. A total of 121 samples of peripheral blood (34 healthy volunteers and 87 patients with RCC) were analyzed in this study. Total RNA of the monocyte fraction of the blood was extracted, then nested RT-PCR using specific primers was performed to detect mRNA of N-cadherin or cadherin-6. Nested RT-PCR revealed that expression of cadherin-6 mRNA was not present in the blood of most healthy volunteers (absent in 32/34), but positive expression was observed in the blood at concentrations of 10 cells/ml or greater of the SKRC-33 RCC cell line, which is a strong expresser of cadherin-6. In peripheral blood from patients with metastatic disease, cadherin-6 mRNA was detected in 70.4% (19/27). Messenger RNA of cadherin-6 was detectable in 45.0% (27/60) of patients with localized tumors. The PCR-based detection system for peripheral blood samples from patients with metastatic disease could reveal the presence of circulating RCC cells in the blood. Detection of cadherin-6 mRNA in non-metastatic presurgical RCC patients suggests that careful follow-up study is necessary in these patients.