Thromboembolic events may be seen in patients with hypertrophic cardiomyopathy (HCM). We investigated the mean platelet volume (MPV), an indicator of platelet activation in patients with HCM. This study included 112 patients with HCM, in which 40 were patients with hypertrophic obstructive cardiomyopathy (HOCM), and 106 were control participants. The MPV was significantly higher in patients with HCM than in controls (9.1 +/- 0.3 vs 7.9 +/- 0.3 fL, P = .01). In the subgroup analyses, MPV was also higher in patients with HOCM compared to those with hypertrophic nonobstructive cardiomyopathy (HNCM; 9.3 +/- 0.3 vs 9.0 +/- 0.2 fL, P = .01). Similarly, patients with HNCM had higher MPV values than controls (9.0 +/- 0.2 vs 7.9 +/- 0.3 fL, P = .01). The MPV was significantly and positively correlated with left ventricular outflow tract (LVOT) obstruction (r = .42, P = .001) and septal thickness (r =.62, P = .001). In linear regression analysis, MPV was independently associated only with septal thickness (beta = .07, 95% confidence interval: 0.04-0.09, P = .001). The MPV can be elevated in patients with HCM regardless of the obstruction of LVOT and may be associated with the severity of septal thickness.