Deep-vein thrombosis of the upper extremity, that is, axillosubclavian vein thrombosis, is a relatively rare and potentially overlooked diagnosis in the emergency department (ED). It can be the cause of significant morbidity and it may be life-threatening. Reported here is the case of a 23-year-old man who presented to the ED with left upper extremity pain and swelling. Venography confirmed the diagnosis of axillosubclavian vein thrombosis. Hospitalization and treatment with intravenous heparin led to a satisfactory recovery. The clinical presentation, predisposing factors, diagnosis, treatment, and complications of this rare entity are discussed.