Study patients Consecutive patients with symptomatic proximal-vein thrombosis who have completed three months of anticoagulation will be eligible for the study. Patients with thrombosis occurring in association with recent (less than three months) trauma, surgical intervention or puerperium, prolonged (more than seven days) immobilization from any cause or use of hormonal treatment will be regarded as "secondary thrombosis". All other patients will defined as having idiopathic thrombosis.