The Fontan operation was initially designed to provide a “physiological correction” of tricuspid atresia, but its indications have been extended to more complex forms of congenitally malformed hearts. In selected patients with two adequately sized ventricles, but highly complex intra cardiac anatomy which makes a biventricular repair more hazardous and which would require multiple reoperations, a univentricular repair using Fontan's principle has been proposed. Between 1978 and 1992, a modified Fontan operation was carried out in 34 such patients. Of the patients. 13 had double outlet right ventricle and 21 had transposed great arteries. Anomalies of ventricular loop were present in 14 patients. A non-committed ventricular septal defect was found in 26 whilst multiple defects were present in 10. All patients had anomalies of the atrioventricular valves, with straddling of the tricuspid valve,11 and/or of the mitral valve11, and abnormal tricuspid5, or mitral4 cordal attachments. Two patients had mild tricuspid hypoplasia, 1 had mitral hypoplasia and 3 had a cleft mitral valve. Fourteen patients had anomalies of both atrioventricular valve but only 6 had mild atrioventricular valvar incompetence. Pulmonary stenosis or atresia was seem in 26. All the patients had at least one or more (mean 2.35) intracardiac anomalies that preculded a biventricular repair. The Fontan operation was performed at a median age of 4.75 years. There were 4 early (11.4%, 70% CL: 5.1–19.8) and 2 late deaths, with a 4 year actuarial survival rate of 79.6±8.1%. Univariate analysis did not reveal any risk factor for early of late death. Follow-up at a mean interval of 3.6 ±2.6 years was available in all but 1 survivor. There were 6 early and 2 late reoperations, and 3 patients required implantation of a pacemaker. In conclusion, the modified Fontan operation can provide satisfactory early and mid term results in patients with two ventricles but with complex intracardiac anatomy which precludes a biventricular repair.