The relationship between Behcet's disease (BD) and platelet aggregation has not sufficiently been investigated yet. Mean platelet volume (MPV) is a marker of platelet function, and the increase in MPV has been identified as an independent risk factor of recurrent vascular events. BD is characterized by a relapsing vasculitis of the venous as well as arterial thrombosis. However, the precise pathogenic mechanisms underlying thrombotic tendency in BD are not known. We hypothesized that there might be an association between thrombotic complication and MPV in these patients. Therefore, we investigated activation of platelets in patients with BD using a simple marker, MPV, the most accurate measure of platelet size. A total of 60 patients with BD and 40 age- and gender-matched controls were included. The BD patients were divided into subgroups based on the presence (n = 22) or absence of thrombosis (n = 38) and clinically active (n = 30) or inactive (n = 30) state. MPV was higher in patients with BD than controls (8.14 +/- 0.8 vs. 7.48 +/- 0.3 fl, p = 0.001). Among BD patients, MPV was larger in patients with thrombosis than those without thrombosis (8.45 +/- 1.0 vs. 7.96 +/- 0.7 fl, p = 0.038). However, there was no significant difference in MPV between BD patients with active and inactive states. The increase in MPV is independent of the disease activity, and the presence of thrombosis is associated with higher MPV in BD patients. Therefore, antiplatelet therapy may be useful to prevent thrombotic complications in BD patients.