Ultrasound-guided thrombin injection is a highly effective therapy for postcatheterization pseudoaneurysm. Despite a very low complication rate, a number of severe arterial thrombotic events have been reported. We present an unusual case of acute femoral vein thrombosis occurring several hours after successful treatment of femoral artery pseudoaneurysm by ultrasound-guided thrombin injection. Pathophysiologic mechanisms are discussed. This case highlights the potential hazards of instilling such a powerful thrombogenic substance in the immediate vicinity of other vascular structures.