This boy had isolated noncompaction of the left ventricular myocardium (also known as persistence of spongy myocardium), a rare form of congenital cardiomyopathy in which the left ventricular wall fails to become flattened and smoother as it normally would during the first 2 months of embryonic development.2,3 This developmental arrest results in decreased cardiac output with subsequent left ventricular hypertrophy. The aberrant left ventricular trabeculae predispose to abnormal cardiac conduction and potentially fatal cardiac arrhythmias. The interstices within the trabeculated left ventricle predispose to thrombus formation with secondary systemic embolic events. Fibroelastosis of the adjacent ventricular endocardium (Figure 3) is a secondary phenomenon resulting from the abnormal blood flow pattern in the left ventricular chamber. No evidence of systemic embolic events was found at autopsy; however, hemosiderin-laden macrophages in the lungs indicate evolving congestive heart failure.