The need for oral anticoagulation in patients with calf-vein thrombosis was examined in a randomised study of 51 patients, of whom 23 received warfarin for 3 months and 28 did not. Both groups received an initial course of heparin and all wore compression stockings. Progress was monitored by the use of serial isotope tests and physical examination. Phlebography was repeated if recurrence was suspected. During the first 3 months, 8 patients in the non-warfarin group (29%) had recurrences compared with none in the warfarin group (p less than 0.01). 5 patients had recurrence with proximal extension and 1 patient had a pulmonary embolus. After 1 year, 22 out of 23 patients in the warfarin group had not had a recurrence, compared with 19 out of 28 (p less than 0.02). The findings indicate that oral anticoagulants should be given to all patients with thrombi that produce symptoms. Treatment for 3 months seems to be sufficient.