Protein Z is a vitamin-K-dependent plasma protein that serves as a cofactor for the inhibition of factor Xa. Although the precise physiologic function of protein Z is still unknown, abnormal plasma protein Z concentrations have been associated with a number of thrombotic disease states. There is the evidence of universal activation of the hemostatic system in Behçet's disease (BD), which represents a hypercoagulable/prothrombotic state. Circulating protein Z levels in patients with BD were evaluated. Plasma protein Z concentrations were assayed in 24 patients with BD (male/female: 13/11, mean age 35.4 years) and in 24 healthy controls (males/females: 14/10, mean age 59.8 years). The disease duration was 10.6 years (range, 1-30 years). None of the subjects in either group had received anticoagulants within 3 weeks before the study, and none of them had liver dysfunction. Patients complicated with vascular disease were also excluded from the study. Mean plasma concentrations of protein Z were 141 ng/mL (range, 56.8-257) in healthy controls and 107.8 ng/mL (range, 21.2-202) in BD patients (p<0.05). There was a positive correlation between the disease duration and protein Z levels in the study group (p<0.05, r=0.448). Alterations of protein Z concentrations could complicate the pathobiology of the prothrombotic state of BD. Furthermore, the tendency of increment in the protein Z with the passage of time may reflect the diminution of the disease activity.