Isolated noncompaction of left ventricular myocardium is a rare disorder of endomyocardial morphogenesis characterized by numerous, excessively prominent ventricular trabeculations and deep intertrabecular recesses. This study comprised eight cases, including three at necropsy. Ages ranged from 11 months to 22.5 years, with follow-up as long as 5 years. Gross morphological severity ranged from moderately abnormal ventricular trabeculations to profoundly abnormal, loosely compacted trabeculations. Echocardiographic images were diagnostic and corresponded to the morphological appearances at necropsy. The depths of the intertrabecular recesses were assessed by a quantitative echocardiographic X-to-Y ratio and were significantly greater than in normal control subjects (p less than 0.001). Clinical manifestations of the disorder included depressed left ventricular systolic function in five patients, ventricular arrhythmias in five, systemic embolization in three, distinctive facial dysmorphism in three, and familial recurrence in four patients. We conclude that isolated noncompaction of left ventricular myocardium is a rare if not unique disorder with characteristic morphological features that can be identified by two-dimensional echocardiography. The incidence of cardiovascular complications is high. The disorder may be associated with facial dysmorphism and familial recurrence.