Left ventricular noncompaction (LVNC) has been recently proposed as a specific form of cardiomyopathy. There have been few pathological series describing gross and microscopic findings of this entity, especially in children. We present findings of 14 hearts (13 autopsy and 1 explant) with LVNC (isolated and associated with congenital heart disease), defined by poorly developed left ventricular (LV) papillary muscles and a noncompact inner LV myocardial layer comprising more than 50% of the LV thickness. The mean age at death/explant was 3.6 years (median, 2.5 months); there were 6 boys and 8 girls. The symptoms were sudden unexpected death (10) and heart failure (4). The diagnosis was suspected before death in only 1 of 13 autopsy cases. Right ventricular involvement (> 75% ventricular thickness comprised of noncompacted area with recess adjacent to tricuspid valve) was seen in 6 of 14 hearts. One patient had a sibling with pulmonary stenosis, but there was no other known familial cardiomyopathy. Endocardial fibroelastosis was a characteristic histological feature, as well as anastomosing or polypoid endocardial trabeculations, which resulted in staghorn-shaped, endocardial-lined recesses. There was a high rate of other cardiac anomalies, which often coexisted and were not clearly related to the LVNC, present in 8 cases (nonisolated LVNC): ventricular septal defect (4/14), anomalous venous pulmonary veins (1/14), coronary ostial stenosis (1/14), histiocytoid cardiomyopathy (1/14), polyvalvar dysplasia (2/14), and pulmonary stenosis (2/14). In the 6 isolated LVNC, there were 2 malformed atrioventricular valves (1 mitral and 1 tricuspid), which appeared part of the ventricular maldevelopment. There were no differences in histological or gross patterns of the noncompacted regions between the isolated and nonisolated LVNC. LVNC is frequently associated with other cardiac defects, especially when causing sudden death in infants and children. A clear-cut morphological distinction between "isolated" and "secondary" LVNC was not apparent. The pathologist should be aware of the entity because the diagnosis is often first established at autopsy.