To evaluate the long-term effects of "conservative" management (heparin initially then Coumadin for 3 months) on patients with axillary vein thrombosis, the authors studied 20 patients (average age 44 years) who presented at the Wellesley Hospital in Toronto between 1975 and 1984. The diagnosis of axillary vein thrombosis was made from history, findings on physical examination and Doppler studies. In 12 patients, the diagnosis was confirmed by venography. Three patients subsequently underwent a first-rib resection for thoracic outlet syndrome. The average follow-up was 42 months. The cause of the thrombosis in 3 patients was an intravenous-line catheter, in 7 it was effort thrombosis and in 10 the cause was unknown. Two patients had had a previous deep venous thrombosis in the lower limb. Results of conservative treatment showed that only five patients had residual minimal swelling and two had minor discomfort. These symptoms did not interfere with either leisure or work activities in any of the patients. Fifteen patients were asymptomatic. One patient had nonfatal pulmonary embolism. The conservative management of axillary vein thrombosis is safe, effective, relatively inexpensive and gives excellent long-term results. The prognosis is good, irrespective of the cause of the thrombosis and, in view of this, a more aggressive approach, using either streptokinase therapy or thrombectomy, does not appear to be justified.