Deep vein thrombosis may occur in the axillary or subclavian veins in the upper extremities, sometimes in young patients without the typical risk factors for thrombus development. This unusual clinical situation usually follows strenuous activity and is often referred to as "effort" thrombosis. Since some cases have occurred with mild exertion and there is a lack of obvious predisposing vessel trauma, a more correct term might be "primary" thrombosis. Early recognition of this condition is necessary to prevent potentially fatal acute effects and minimize chronic sequelae.