Isolated noncompaction of the ventricular myocardium (INVM) was diagnosed with delayed enhancement cardiac magnetic resonance imaging (MRI) in a 12-year-old boy, who developed dyspnea and syncope while running. Chest radiograph showed no marked cardiomegaly, but revealed bilateral consolidation caused by aspiration pneumonia. Laboratory findings showed plasma level of brain natriuretic peptide (BNP) of 768 pg/dl. Echocardiography showed a slightly thickened myocardium, but the trabecular meshwork region was unclear. He was given a diagnosis of unknown heart failure. His dyspnea and cyanosis improved in response to inotropic agents, oxygen and steroid therapy. However, the plasma BNP levels could not be decreased to normal. Cardiac MRI was performed and delayed-enhancement demonstrated hyperenhancement of prominent trabeculation in the lateral and apical regions of the left ventricle, suggesting fibrosis. The patient was given a diagnosis of INVM, but his status was New York Heart Association Class I heart failure. The diagnosis of INVM in children is difficult because heart failure symptoms are present in only 30% of the cases at diagnosis. Delayed-enhancement MRI is a more precise method of assessment.