Axillary–subclavian venous thrombosis (ASVT) is classified as either primary, which is often associated with active use of the upper extremities and underlying anatomic abnormalities, or secondary to recognized thrombotic risk factors. Although there is a risk of pulmonary embolism with both, late outcome varies with etiology. Primary ASVT occurs in young healthy patients, and up to 80% may have significant chronic symptoms. Secondary ASVT occurs in older patients with medical comorbidities, high mortality rates, and few late manifestations. Although anticoagulation is appropriate for most patients with secondary ASVT, a multidisciplinary approach including catheter-directed thrombolysis, thoracic outlet decompression, and correction of intrinsic venous lesions more effectively prevents late symptoms in selected patients with primary ASVT.