We report an exceptional case of a 12-year-old-boy with a pseudoaneurysm of the popliteal artery caused by an osteochondroma on the lower femoral metaphysic after a hyperextension trauma. On account of the absence of the most constant findings, the pulsation of a soft tissue mass and elevated inflammation blood parameters, we preoperatively assumed an infected haematoma. This patient was successfully operated upon with resection of the osteochondroma and saphenous vein femoropopliteal bypass. Owing to the severity of potential vascular complication, preventive resection of osteochondromas in the course of an artery and the need for a vascular surgeon on stand-by should be discussed.