ABSTRACT. The length of time for which deep vein thrombosis (DVT) should be treated with oral anticoagulants (OA) is controversial. In this study, 135 patients with symptomatic first period DVT (83 % with proximal DVT) were randomly allocated to OA for one or six months. The diagnosis of initial and recurrent DVT was confirmed by phlebography or plethysmography and thermography, or by a combination of all these methods. Pulmonary emboli were confirmed by lung scans or at autopsy. The patients were followed for at least one year. One patient had to discontinue OA prematurely because of haemorrhage. Seventeen patients left the project for other reasons, ten during and seven after therapy; in one of these DVT recurred. The recurrence rate during the first year was high (17% symptomatic recurrences) irrespective of whether OA had been given for one or six months.