A thromboembolic event in patients later given a diagnosis of cancer is the result rather than the cause of the cancer. The risk of hidden cancer is significantly higher for patients with recurrent idiopathic thromboembolism compared to those with secondary deep vein thrombosis. Microemboli from hepatic or adrenal metastases and large-sized emboli from the great veins invaded by the tumor are the sources of tumor embolization. The intraarterial tumor emboli less likely invade the arterial wall. Thrombus formation and organization may be capable of destroying tumor cells within pulmonary blood vessels. Therefore, all tumor emboli are not true metastases. The treatment of deep vein thrombosis and pulmonary embolism in patients with cancer consists of anticoagulation with heparin and warfarin, venacaval filters, appropriate anti-neoplastic agents, and surgical methods(embolectomy, thromboendarterectomy). However, considerable literatures suggest that oral anticoagulant such as warfarin is ineffective in the treatment of those. We report a case of primary unknown squamous cell carcinoma incidentally found in the thrombus after pulmonary embolectomy.