Previous studies have demonstrated that platelet activation occurs in patients with acute pulmonary embolism (PE). The aim of this study was to assess the mean platelet volume (MPV), an indicator of platelet activation in patients with acute PE. The study group consisted of 107 patients with acute PE. Seventy subjects matched for age, gender, body mass index (BMI) and frequency of concomitant diseases served as control group. All patients and control subjects underwent physical examination and echocardiography. We measured MPV values and platelet counts on admission. MPV was significantly higher among patients with acute PE when compared with control group (9.6 ± 1.0 vs. 8.1 ± 0.8 fL respectively; < 0.001). Platelet count was significantly lower among acute PE patients when compared with control group (227.1 ± 77.0 vs. 268.7 ± 58.4 × 10(9)/L, respectively; < 0.001). MPV was correlated with right ventricular (RV) diameter (p < 0.001, r = 0.33) in correlation analysis. In linear regression analysis, MPV was independently correlated with RV dimension (β = 0.29, p = 0.001). We have shown that MPV, an indicator of platelet activation, was increased in patients with acute PE and it was correlated with RV diameter. Platelet count was decreased in patients with acute PE.