Objective: To study and verify the hypothesis that circulating tumor cells can be detected in peripheral blood of patients with NSCLC by reverse transcriptase polymerase chain reaction (RT-PCR) of carcinoembryonic antigen (CEA) mRNA and by tissue immunohistochemical staining. Also, to evaluate the possible effect of different interventional procedures in dissemination of tumor cells. Methods: Peripheral blood samples were taken pre and again one and two weeks post to interventional procedures from 40 patients with NSCLC who were recently diagnosed and assayed for CEA mRNA by RT-PCR. Blood samples taken from 4 cases with SCLC and 16 patients with non malignant chest diseases served as controls. Results: No control samples were positive for CEA by RT-PCR except for the 4 cases with SCLC who gave positive results in the pre and postinterventional blood samples. Twenty seven (67.5%) of the preinterventional blood samples from patients with NSCLC were positive whereas 13 (32.5%) were negative. The postinterventional blood samples were positive in all the previously preinterventional positive cases. Also, all the negative cases in the preinterventional blood samples continued to be negative except for 4 cases changed from negative to positive results. The sensitivity, specificity, positive predictive and negative predictive values of CEA mRNA in detecting metastasis in NSCLC were 100%. Conclusions. The study provides a valuable evidence of the presence of occult tumor cells in the peripheral blood of unexpected large number of NSCLC patients. The change from negative preinterventional results to positive postinterventional results should draw attention to the possible danger of tumor dissemination during the diagnostic procedures.