Pseudoaneurysms arise from a disruption of arterial wall continuity and are most commonly related to a penetrating trauma, an arterial wall inflammation or iatrogenic causes. They differ from real aneurysms due to a lack of one or more layers of the arterial wall. The frequency of peripheral artery pseudoaneurysms in the upper extremities is less than in the lower extremities and its most common cause is a gunshot or a stab wound. The risk of a rupture is higher than in true aneurysms due to a lack of wall layers, therefore requiring surgical treatment in most cases. Here we describe an unusual case of an 8-year-old girl who presented to the emergency department complaining of swelling and pain in her left distal forearm. One month before admission she experienced a penetrating trauma in the same area due to a self inflicted stab wound. After clinical and duplex ultrasonography evaluation the tumefaction proved to be a posttraumatic pseudoaneurysm of the left radial artery.