Leiomyosarcomas are rare malignant mesenchymal tumors. We report the case of a 55-year-old man with a lung metastasis from an iliofemoral leiomyosarcoma mimicking a deep venous thrombosis (DVT). Vascular leiomyosarcoma occurs in the wall of large vessels, and usually presents as unexplained DVT. To date, few cases of peripheral leiomyosarcomas have been described in the literature. This observation demonstrates the usefulness of Positron Emission Tomography (PET), beyond oncologic staging, in such indications as the workup of an unexplained DVT. In this report, PET prevented a misdiagnosis of DVT by showing a peculiar cockade appearance of the vessel. Compared with conventional imaging, PET can provide complementary information. The specificity of such information still needs to be assessed.