To assess the prognostic significance of myocardial contractile reserve in patients with idiopathic dilated cardiomyopathy (DCM) without overt heart failure (New York Heart Association functional class I or II), seventy-one patients underwent exercise radionuclide angiography in addition to clinical, radiographic, hemodynamic, and echocardiographic evaluations. Myocardial contractile reserve was assessed as left ventricular ejection fraction (LVEF) during peak exercise minus LVEF at rest (ΔLVEF). During an average of 49 months, 18 patients died of the disease. Cox's proportional-hazards regression analysis showed that the ΔLVEF was the most powerful and independent discriminator for survival (p=0.0002). Ejection time (p=0.0029) and cardiothoracic ratio (p=0.017) were the second and third most predictive variables, respectively. Evaluation of the ΔLVEF, which reflects residual myocardial contractile reserve, can provide important information about the prognosis of patients with DCM and mild symptoms.