Unbalanced atrioventricular septal defect is seen by 10-15% of the patients with this congenital heart disease and poses serious challenges for the treating physicians regarding the choice of the most suitable operative intervention. The forms with moderate degree of unbalance between the two ventricles, especially when the size of the left ventricle is considered borderline, make the choice between two-ventricle correction and single-ventricle palliation quite difficult. The most important factor when judging the degree of unbalance is not the size of the ventricles but rather the redistribution of blood flow. The use of different echocardiographic measurements could contribute to the more precise determination of the indications for one of the two surgical strategies with resulting improvement of patient outcome.