Of 500 patients who had a modified Fontan operation at this institution between 1973 and 1987, 54 (33 boys and 21 girls) were <4 years old. This retrospective study related preoperative clinical and hemodynamic data to subsequent survival. Twenty patients <4 years old had tricuspid atresia, 13 had double inlet ventricle and 21 had other complex heart defects. There were 14 early deaths (<30 days after operation) and 6 late deaths. Multivariate analysis of survival for the entire group of 500 patients revealed the following factors to be significantly associated with poorer survival: absence of tricuspid atresia (p = 0.011), asplenia (p < 0.001), age <4 years at operation (p = 0.042), atrioventricular valve dysfunction (p = 0.017), early calendar year of operation (p < 0.001) and the presence of either one or more of the following: left ventricular ejection fraction <60%, mean pulmonary artery pressure <15 mm Hg and pulmonary arteriolar resistance <4 U·m2 (p < 0.001). On the basis of this study of 500 patients, age <4 years at operation appears to be an independent risk factor for poorer survival after the modified Fontan operation.