Systemic thromboembolism is a serious major complication in patients with mitral stenosis. However, the pathogenesis of thromboembolism in mitral stenosis is not fully understood. Previous studies have demonstrated that platelet activation occurs in serum of patients with rheumatic mitral stenosis (MS). The aim of this study was to assess the mean platelet volume (MPV), an indicator of platelet activation, in patients with MS. The study group consisted of 36 patients with MS who were in sinus rhythm. An age and gender matched control group was composed of 30 healthy volunteers. We measured serum MPV values in patients and control subjects. MPV was significantly higher among MS patients with sinus rhythm (SR) when compared with the control group (9.2+/-1.4 vs 8.1+/-0.9 fl respectively; p<0.001). We have shown that MPV was significantly elevated in patients with MS who were in SR compared to control subjects.