The possible influence of small pulmonary artery size on the results following complete repair of tetralogy of Fallot has not been investigated in patients less than 1 year of age. We, therefore, reviewed our recent experience with infant repair to assess this. Between January 87 and October 90, repair was performed in 58 infants less than 1 year old (median, 6 months) with tetralogy of Fallot and pulmonary stenosis (n = 53) or pulmonary atresia without important aortopulmonary collaterals (n = 5). The McGoon ratio (diameter of the right pulmonary artery at the prebranching point plus that of the left divided by the diameter of the aorta at the diaphragm) by cineangiography (n = 48) was between 1.2 and 1.5 in 15 patients, 1.6 and 2.0 in 20, and greater than 2.0 in 13. Transannular patching was performed in 19 of the patients with pulmonary stenosis (36%). Muscle resection was avoided in 29 infants and was minimal in the remaining 29. A weak correlation, with considerable scatter, was found between the McGoon ratio and postrepair right ventricular/left ventricular pressure measured in the operating room (r = 0.4), and the relations were similar to those previously reported for children. Right ventricular/left ventricular pressure was 0.55 +/- 0.03 in infants with McGoon ratios of 1.2-1.5, 0.46 +/- 0.03 with ratios of 1.6-2.0, and 0.47 +/- 0.03 with ratios greater than 2.0 (p = 0.01). There were no hospital deaths, and the 3-year survivorship was 94%. Two infants underwent successful reoperation for residual right ventricular outflow tract obstruction. All other patients are free of significant hemodynamic residua.(ABSTRACT TRUNCATED AT 250 WORDS)