Attenuated coronary flow reserve (CFR) has been reported in patients with idiopathic dilated cardiomyopathy (IDC). On the other hand increased platelet activity has been demonstrated in patients with congestive heart failure and left ventricular dysfunction. Accordingly, we aimed to investigate whether mean platelet volume (MPV) is increased in patients with IDC and increased MPV correlates with the degree of coronary microvascular dysfunction.