Trisomy 21 accounts for 3 p. 100 of reasons for admission to the Paediatric Cardiology unit of the St Luc University Clinics, Brussels. In a series of 142 cardiac children with trisomy 21 evaluated by catheterization between 1969 and 1987, 54 p. 100 of the cardiac malformations observed consisted of persistent common atrioventricular canal (complete in 45 p. 100 of the cases). The other heart diseases were ventricular septal defect (23 p. 100), atrial septal defect of the ostium secundum type (10 p. 100) and tetralogy of Fallot (9 p. 100). In 40 p. 100 of the patients other cardiovascular abnormalities were associated with these predominant intracardiac shunts. These findings were in agreement with those usually reported in the literature. At the time of investigation (mean age 24 months), pulmonary vascular resistance had already reached a pathological level in 88 p. 100 of the cases. Oxygen tests only slightly improved these results, which suggested that the conditions were favourable to the early development of a pulmonary obstructive vascular disease in Down's syndrome, thus darkening the prognosis of congenital heart disease in mongoloid children. In this series to overall mortality rate of corrective surgery was 23 p. 100. The risk was maximum in infants aged less than 3 months with severe and rapidly symptomatic lesions. The outcome in patients successfully operated upon was satisfactory, with benign residual lesions.(ABSTRACT TRUNCATED AT 250 WORDS)