Background: Pulmonary thromboembolism is a serious complication in patients with deep vein thrombosis (DVT). Previous studies have demonstrated that platelet activation occurs in patients with DVT. Mean platelet volume (MPV) is a simple and easy method of assessing platelet function. The aim of this study was to assess the MPV, an indicator of platelet activation in patients with DVT. Patients and Methods: The study group consisted of 52 patients with newly diagnosed acute DVT. An age-, gender-, and body mass index-matched control group consisted of 30 healthy volunteers. All patients and control participants underwent ultrasonographic examination. We measured the serum MPV values in patients with DVT and control participants. Results: Mean platelet volume was significantly higher among patients with DVT when compared with the control group (8.6 +/- 0.8 vs 7.7 +/- 0.9 fL, respectively; P < .001). Conclusion: We have shown that MPV was significantly elevated in patients with DVT compared to controls.