Little is known about the implications of left ventricular noncompaction in children with additional congenital cardiac malformations. With this in mind, we conducted a retrospective review of every left ventricular angiogram performed in a single tertiary referral centre for paediatric cardiology, in Melbourne, Australia, between 1994 and 2000 in children with congenital heart disease, looking specifically for patients with angiographic evidence of noncompaction of the left ventricle. The outcome of patients identified as having noncompaction was compared with that of patients from the same population, stratified by their primary congenital cardiac malformation. Of 1515 children undergoing left ventricular angiography, 31, with 13 being male, were found to have angiographic evidence of left ventricular noncompaction, giving a prevalence of 2% (95% CI: 1.3%-2.8%). Of 69 (22%) children with a functionally single left ventricle, 15 fulfilled the criterions for noncompaction, compared to 16 of 1446 (1.1%) children with a balanced ventricular arrangement (p is less than 0.0001). The presence of noncompaction and a functionally single left ventricle were each associated with a doubling of mortality, the effect being cumulative. Of surviving patients with left ventricular noncompaction, 19% (4%-34%) have left ventricular dysfunction at their latest follow-up. We suggest that the important late sequels of noncompaction justify careful scrutiny for this entity in children with congenital cardiac disease.