To test the hypothesis that subclinical levels of ventricular dysfunction contribute to the development of ventricular arrhythmias after repair of tetralogy of Fallot, 38 postoperative patients were studied by radionuclide ventriculography and M-mode echocardiography. Eighteen patients (group I) had Lown grade 2 or greater ventricular arrhythmias on ambulatory electrocardiography or treadmill exercise, or both; 20 patients (group II) had no documented ventricular arrhythmias. Radionuclide ventriculograms were performed using technetium -99m-labeled red cells; ejection fractions were derived by computer from multigated images, with normal values being 45% for the right ventricle and 55% for the left ventricle. From M-mode echocardiography, right and left ventricular end-diastolic dimensions were expressed as a ratio, the highest normal value being 0.45. By radionuclide ventriculography, right ventricular ejection fraction was lower for group I (28 +/- 3%) than for group II (31 +/- 2%), but the difference was not significant (p less than 0.10). Left ventricular ejection fraction was significantly lower for group I than for group II (45 +/- 5% versus 55 +/- 3%, p less than 0.05). The echocardiographic right and left ventricular diastolic dimension ratio was elevated in all patients except two in group II; it was significantly greater in group I than in group II (0.84 +/- 0.06 versus 0.63 +/- 0.04, p less than 0.005). This study provides evidence for right ventricular dilation by M-mode echocardiography and for biventricular dysfunction by radionuclide ventriculography in patients who have undergone repair of tetralogy of Fallot.(ABSTRACT TRUNCATED AT 250 WORDS)