We report a case of pseudoaneurysm of the cephalic vein which was diagnosed in a 13-year-old boy who presented with a superficial mass in the right upper arm. The patient had no history of trauma, venipuncture, or intravenous drug addiction. Ultrasonic examination revealed a defect in the anterior wall of the cephalic vein and a surrounding vascular pool which included the inner thrombus. The mass was easily peeled off and completely excised after ligation of the afferent and efferent cephalic vein. Pathologic examination revealed it to be a pseudoaneurysm of the cephalic vein. This may represent the first case of venous pseudoaneurysm in the English-language medical literature without causative preceding events. Clinical features, including etiology, diagnosis, treatment, and complications, of this extremely rare lesion are also discussed.