The present report discusses a case of noncompaction of the ventricular myocardium in a 51-year-old woman with dilated cardiomyopathy of unknown etiology. Multiple transthoracic echocardiography examinations had failed to show myocardial noncompaction, but subsequently performed transesophageal echocardiography clearly demonstrated the characteristic findings of this unusual disease. Also, an atrial septal aneurysm was identified by transesophageal echocardiography. Patients with cardiomyopathy Of unknown origin should be investigated to define the presence or absence of myocardial noncompaction, even if transthoracic echocardiography fails to show anatomical features of this disease. The present case is the first reported case of ventricular noncompaction associated with an atrial septal aneurysm as a congenital anomaly.