Patients with first venous thromboembolism (VTE) and high factor VIII (FVIII) are at increased risk of recurrence. It is unknown whether these patients benefit from prolonged secondary thrombophrophylaxis. In a prospective trial patients with first spontaneous VTE and FVIII levels >230 IU/dl were randomized to discontinue vitamin K Antagonist (VKA) after 6 months or to continue VKA for additional 24 months. Patients were excluded if they had a natural inhibitor deficiency, lupus anticoagulant, cancer, were pregnant, required long-term antithrombotic therapy or had acute-phase reaction. Primary study endpoints were symptomatic recurrent VTE or major bleeding within 2 years. Follow-up was continued beyond 2 years. Of 3,219 screened patients 34 met the inclusion criteria. Mean observation time was 37 months. Two of 17 patients allocated to discontinue VKA and two of 17 patients randomized to prolonged anticoagulation had recurrent VTE within 2 years. In the prolonged treatment group, one patient had recurrence during VKA therapy and one patient 4 weeks after voluntary discontinuation of VKA. One major nonfatal bleeding (severe epistaxis) after 10 months of VKA occurred in the prolonged treatment group. Five patients allocated to prolonged anticoagulation had recurrent VTE after discontinuation of VKA. The probability of recurrence at 2 years after discontinuation of VKA was 30% (95% CI 13-46%). Patients with high FVIII are at increased risk of recurrence. Our findings in a small number of patients indicate that prolonged anticoagulation seems to be effective but that the benefit is not maintained after discontinuation of anticoagulation.