Vascular complications that are the result of invasive diagnostic or therapeutic procedures requiring arterial access occur frequently. A common complication, pseudoaneurysm, has significant risk of expansion, extrinsic compression on native arteries, rupture, embolization, and infection. A pseudoaneurysm represents a persistent defect in the walls of the artery, resulting in extravasation of blood outside of the artery. This extravascular blood is confined to the surrounding soft tissues. Clinical suspicion and duplex ultrasonography are the primary diagnostic modalities. There are several options for treatment of pseudoaneurysms, including surgical repair and ultrasound-guided compression. However, a new therapeutic option, ultrasound-guided thrombin injection, has become the treatment of choice for pseudoaneurysms. This procedure is highly effective, rapid, and very safe. Using direct, real-time visualization of the pseudoaneurysm, a needle is inserted percutaneously into the pseudoaneurysm sac, and a small amount of dilute bovine thrombin is injected. Thrombosis of the sac is rapidly evident. Rare complications include thromboembolism into the native artery and hypersensitivity reaction to bovine thrombin.