A leiomyosarcoma was detected in the region of the left iliac vein in a 37-year-old male patient who had experienced intermittent swelling of the left leg for a period of 3 months prior to admission. The results of the diagnostic imaging procedures carried out (ultrasonography, CT scan, phlebography) all pointed to a thrombus affecting the iliac vein. None of the other diagnostic procedures employed provided any evidence of a malignant disease. The definitive diagnosis was made only on the basis of the histological workup of the material obtained during surgery. The patient died 22 months after establishment of the initial diagnosis with hepatic, pulmonary, renal, and splenic metastatic disease having developed in the meantime. Treatment with chemotherapy and hyperthermia failed to prevent this fatal outcome. This case shows that, in the presence of atypically located “thrombi,” differential diagnostic considerations prior to thrombolytic treatment and surgery should include a tumor, particularly when previous thrombolytic treatment has proved ineffective.