Noncompaction refers to an uncommon structural abnormality of the heart's ventricular myocardium characterized by an abnormally thick layer of left ventricular trabeculations, as well as hypoplastic papillary muscles. The condition is associated with a variable clinical phenotype including heart failure, thromboembolism, and sudden death. In this retrospective study of fetal and neonatal autopsy hearts with noncompaction, clinical profiles were correlated with gross and histologic findings and compared with a set of age-matched controls. Pathologic criteria for noncompaction included hypoplastic left ventricular papillary muscles, abnormal trabecular architecture and greater than 50% penetration of the left ventricular wall thickness by intertrabecular recesses. Among eight fetuses and full-term neonates with pathologic features of noncompaction, all had evidence of severe heart failure, including four with complete heart block and two others with bradycardia. None experienced sudden death. Seven of eight hearts had associated heart malformations—four with left atrial isomerism, two with aortic and/or pulmonary valve dysplasia consistent with stenosis and, one with atrial septal defect. One heart with noncompaction had no associated malformations. With characteristic excessive trabeculation in the left ventricle, noncompaction also included biventricular endocardial fibroelastosis that denoted right ventricular involvement in all hearts. Thus, (1) among autopsied fetuses and neonates with noncompaction, heart failure including heart block is a common cause of death, (2) noncompaction is often associated with various cardiovascular malformations, but even in isolation it can be the basis for severe cardiac failure, and (3) biventricular endocardial fibroelastosis in noncompaction suggests a global pathologic process. © 2013 Wiley Periodicals, Inc.