Complete atrial ventricular septal defects (AV canal) generally have right and left valve components equally divided. However, in unbalanced AV canal either right or left ventricle dominance may occur. The spectrum may vary between those readily able to undergo biventricular repair at no increased risk to those requiring a single ventricle approach in cases with severe hypoplasia of the ipsilateral ventricle. The most challenging diagnostic cases fall within the gray area between the two ends of the spectrum, in which one ventricle is not clearly hypoplastic. Diagnostic modalities that are used to evaluate these ventricles include echocardiography and angiographic ventriculography using volume formulae. Magnetic resonance imaging (MRI) has also recently been found to be a useful technique with which further experience is being developed. Once the determination of a single ventricle physiology has been made, early intervention (ie, pulmonary artery banding) to protect both the pulmonary vasculature and the ventricular function is performed. Follow-up with catheterization and bidirectional Glenn should be completed usually by around 4 to 6 months of age. This palliation and subsequent Fontan completion has had good results based largely on two factors: ventricular dominance, left being better than right, and the absence of pulmonary vascular hypertension.