This study was undertaken to assess the value of percutaneous dilatation of the right ventricular outflow tract as a substitute for surgical systemic pulmonary anastomosis in varieties of tetralogy of Fallot with severe irregularity or major pulmonary arterial hypoplasia unsuitable for complete repair of first intention. Fifteen neonates aged 3 to 23 days with severe desaturation (SaO2 = 73 +/- 11%) and twenty five children aged 1.2 to 174 months with anoxic crises or severe desaturation (SaO2 = 67 +/- 15%) fulfilled these criteria. They underwent right heart catheterisation completed by an attempted pulmonary dilatation. This manoeuvre failed in 6 cases, all neonates, because it was not possible to cross the annulus or maintain the balloon in position, or because of infundibular perforation. There were complications in 5 cases, all neonates: 2 tamponades with 1 death, one dissecting aneurysms of the annulus, 1 gastroenteritis and 1 caval thrombosis. Of the 34 patients dilated, 8 were poor results, mainly in the older age group (7 cases) and 26 were successful as judged by a significant improvement in arterial saturation. However, 7 patients rapidly deteriorated due to anoxic crises (6 infants and children) or fatal secondary tamponade (1 neonate). Therefore, there remained 19 stable successful procedures, 49% of attempts, representing 56% of successful attempts and 73% of primary successes. In neonates, these proportions were respectively: 47%, 78% and 87%, and in the older children: 48%, 48% and 63%.(ABSTRACT TRUNCATED AT 250 WORDS)