Two anticoagulant regimens, similar except for the timing of warfarin therapy, were compared in patients with clinically submassive venous thromboembolism (VTE). Warfarin was begun after 7 days of continuous intravenous heparin infusion in group L (127 patients) or within 3 days (average 1 day) of starting heparin in group S (139 patients), with similar outcomes. The frequency of symptomatic VTE recurrence during the hospital stay was 4.7% in group L and 3.6% in group S, and that of symptomless new perfusion defects 8.5% in group L and 3.9% in group S. On routine iodine-125-fibrinogen leg scanning of patients presenting with distal thrombosis (in the calf, popliteal, or distal femoral veins) 3.6% of group S but no group L patients had symptomless proximal extension. The incidence of bleeding was similar with both regimens. Outpatient follow-up showed no excess recurrent VTE in either treatment group. Early warfarin treatment significantly shortened hospital stay by an average of 3.9 days (30%) in patients admitted solely because of VTE.