Isolated noncompaction of the ventricular myocardium is a rare congenital cardiomyopathy resulting from an arrest in normal endomyocardial embryogenesis. The diagnosis can be made echocardiographically, and the entity may be associated with problems of cardiac rhythm. We describe two illustrative cases, with Wolf-Parkinson-White syndrome in one, and left bundle branch block in the other. We emphasize that children with problems of rhythm and findings suggestive for left ventricular cardiomyopathy, either hypertrophic or dilated; should be investigated to exclude isolated noncompaction of the ventricular myocardium.