Aim: A “seed” of lung cancer metastasis is circulating tumor cells (CTCs), which may be dislodged from a tumor during biopsy. This possibility was assessed among patients who underwent lung tumor biopsy using flexible fiber-topic bronchoscopy (FFB). Methods: The study involved six patients with non-small cell lung cancer who underwent FFB biopsy to diagnose a lesion pathologically (5 males and 1 female, median age 63 years, 6 adenocarcinomas, of 4 clinical-stage IA, 1 stage IB, and 1 stage IIIA), CTCs were extracted from the peripheral vein blood at pre-FFB and at post-FFB using a size selection method. Results: No tumor cell was detected at pre- and post-FFB was in three cases (50%); no tumor cells were detected pre-FFB while CTCs were detected at post-FFB in two cases (33.3%); and CTCs were detected at pre-FFB with numerous CTCs detected at post-FFB in one case (17.7%). In addition, similar tendencies were observed in each analysis of single-cell and clustered-cell categories. Conclusion: These results suggest that a FFB biopsy of lung cancer may potentially dislodge CTCs from a tumor into the circulating peripheral blood.