We describe a new technique for clotting aneurysms by direct injection of diluted thrombin through a fine needle or cannula. The four patients treated had pseudoaneurysms of the iliac, femoral, and peroneal arteries, and a true aneurysm of an accessory hepatic artery. Five hundred to one thousand (500-1000) units of thrombin were injected into the aneurysmal sac only when there was blood stasis, either de novo or induced by occlusion of the feeding vessel. No significant complications ensued.