Approximately 50% of patients with Trisomy 21 have congenital heart lesions and they account for 2.7% of the admissions to the Pediatric Cardiac Unit of the Cardiac Hospital of Lyons. In a series of 91 cases observed over a 10 year period (1070-1980) undergoing hemodynamic and angiographic investigation, 9 out of 10 cases - as previously reported - had the following abnormalities: endocardial cushion defect, 42.7% (39% of which were complete atrioventricular canals), ventricular septal defect, 33%, and Fallot's tetralogy, 12%. These lesions were characterised by the high incidence and precocity of pulmonary hypertension (69 out of 72 cases with a left-to-right shunt . A comparison of the hemodynamic data of children with Trisomy 21 with ventricular septal defects, and children without Trisomy 21 of the same age with ventricular septal defect showed the pulmonary arterial resistances to be significantly higher in the group with Trisomy 21, suggesting that Trisomy 21 plays a role in the pathogenesis of severe early pulmonary hypertension. The operative risk is much higher in these patients, especially with openheart surgery (4 operative deaths out of 12 cases of closure of VSD). The surgical indications are difficult to define because of the context of malformation and mental retardation, the high incidence of pulmonary hypertension and the high opertive risk requiring a large and detailed dialogue with the family.