Primary vascular leiomyosarcoma is a rare malignant tumor arising from the smooth muscle cells of the media on the vessel wall. We describe here the ultrasonography (US) and magnetic resonance (MR) findings, as well as pathologic correlation, of a leiomyosarcoma of the femoral vein in a 42-year-old woman. US revealed a spindle-shaped mass with a connection to the thrombosed femoral vein. We initially misdiagnosed this mass as a peripheral nerve sheath tumor due to the misinterpretation of the thrombosed vein as a nerve bundle.