The reverse transcriptase-polymerase chain reaction (RT-PCR) of tumor-specific or -associated genes is a sensitive assay for detecting a minimal number of tumor cells in peripheral blood (PB) or bone marrow (BM). In this study, we determined whether mRNA of bombesin receptors is detectable in PB or peripheral blood progenitor cell (PBPC) samples from patients with small cell lung cancer. Among three bombesin-like peptide receptors, we used the neuromedin B receptor (NMB-R) gene as a target, because of the most frequent expression on SCLC cell lines. The lower limit of detection was one tumor cell in one million normal PB cells and there was no detection in normal PB or BM cells unlike a cytokeratin 19 gene. The NMB-R gene was detected in 14 (31.8%) of 44 PB samples from patients with SCLC at diagnosis and 2 (15.4%) of 13 samples of PBPC collected during a recovery phase after chemotherapy followed by administration of G-CSF (filgrastim). At diagnosis, patients whose PB was positive for the NMB-R gene had a significantly shorter survival than those who were negative. Our observation suggests that this assay may be useful in diagnosing metastatic disease and monitoring minimal residual disease in patients with SCLC.