Internal jugular vein thrombosis as first sign of metastatic lung cancer. We describe a case of spontaneous internal jugular vein thrombosis occurring as the first sign of occult lung cancer. The peculiarity of the case was the unusual site of thrombosis and the lack of risk factors for DVT (only a moderated reduction of protein S without inherited thrombophilia) as well as the absence of clinical signs of cancer. This report shows once again the strong association between idiopathic venous thromboembolism and cancer. Hypercoagulation tests confirmed the association between cancer and thrombophilia. Reduction of protein S has been discovered recently in patients with lung cancer but further data are required to confirm this finding. Combined treatment (chemotherapy and radiotherapy) associated with oral anticoagulation therapy was started in our patient at the moment of diagnosis.