Purpose: Tumor cells from lung cancer patients are expelled from the primary tumor into the blood, but difficult to detect in the peripheral circulation. We studied the release of circulating tumor cells (CTC) during surgery to test the hypothesis that CTC counts are influenced by hemodynamic changes (caused by surgical approach) and manipulation. Experimental Design: Patients undergoing video-assisted thoracic surgery (VATS) or open surgery for (suspected) primary lung cancer were included. Blood samples were taken before surgery (T0) from the radial artery (RA), from both the RA and pulmonary vein (PV) when the PV was located (T1) and when either the pulmonary artery (T2 open) or the PV (T2 VATS) was dissected. The CTC were enumerated using the CellSearch system. Single-cell whole genome sequencing was performed on isolated CTC for aneuploidy. Results: CTC were detected in 58/138 samples (42%) of 31 patients. CTC were more often detected in the PV (70%) compared to the RA (22%, p<0.01) and in higher counts (p<0.01). After surgery, the RA but not the PV showed less often CTC (p=0.02). Type of surgery did not influence CTC release. Only 6/496 isolated CTC showed aneuploidy, despite matched primary tumor tissue being aneuploid. Euploid so-called CTC had a different morphology than aneuploid. Conclusion: CTC defined by CellSearch were identified more often and in higher numbers in the PV compared to the RA, suggesting central clearance. The majority of cells in the PV were normal epithelial cells and outnumbered CTC. Release of CTC was not influenced by surgical approach.