We investigated activated protein C resistance (aPCR) using modified activated partial thromboplastin time (aPTT) in 32 patients with Behçet's disease (BD) and 9 healthy controls. None of the healthy controls were found to have aPCR. However, 11 out of 32 Behçet's patients (34.3%) were found to have aPCR. The frequency of aPCR was increased to 44.4% among 18 Behçet's patients having a history of venous thrombosis. In the subgroup of 14 patients without venous thrombosis, aPCR frequency was %22.2. Our findings show that, besides other factors, aPCR may also predispose patients to venous thrombosis in BD. The detection of aPCR, using modified aPTT may serve as a routine screening test to determine the necessity of prophylactic anticoagulation reatment in patients with BD.