Cancer is known to promote its own development/proliferation and protect itself against attacks from immune system cells by activating the blood coagulation system. However, antithrombotic therapy inhibits the blood coagulation system. We investigated the blood coagulation system-mediated influence of preoperative antithrombotic therapy on the clinicopathological parameters of lung cancer. In patients who underwent antithrombotic therapy, there was a significant association between prothrombin time-international normalized ratio (PT-INR and positive findings on a thoracic lavage cytodiagnosis (p = 0.02). In these patients, the proportion of those with positive findings on a thoracic lavage cytodiagnosis was significantly higher than in those who did not undergo antithrombotic therapy (P = 0.0003). These results suggest that cancer progression is promoted by antithrombotic therapy through the inhibition of the blood coagulation system.