OBJECTIVES: Vasovagal syncope (VVS) is supposed to be modulated by increased sympathetic tone following an orthostatic maneuver. Increased sympathetic activity may have an important role in mean platelet volume (MPV), either by peripheral activation or by effects on thrombocytopoiesis. We aimed to show the effects of increased sympathetic activity on platelet size in patients with VVS in the present study. PATIENTS AND METHODS: Thirty-seven patients with VVS were compared with age- and sex-matched 33 patients without VVS. All patients have undergone 24 hour holter monitoring for heart rate variability (HRV) and time-domain HRV analysis. Blood samples for MPV measurements were taken before 24 hour holter monitoring. RESULTS: Group 1 was consisted of 37 patients with VVS and group 2 was consisted of 33 patients without VVS. We observed that SDNN, SDNN index, SDSD, RMSDD, PNN50 count were significantly lower and MPV was found significantly higher in patients with VVS (p < 0.05 for all). Pearson\'s correlation analysis showed that MPV was moderately negatively correlated with SDNN (r = -0.421), SDSD (r = -0.396), NN50 count (r = -0.395) and RMSDD (r = -0.393). Multivariate regression analysis showed that SDNN was the only independent variable, which had a significant effect on increased MPV level (beta = -0.295 , p = 0.016). CONCLUSIONS: We found that MPV was closely associated with increased sympathetic activity in patients with VVS. Our analysis supports the hypothesis that alterations in autonomic status might play a role in the development of platelet size.