The magnitude of left-to-right shunt in 55 children with isolated congenital heart disease [atrial septal defect (ASD) or ventricular septal defect (VSD) (muscular and perimembranous)] was estimated by two methods: radionuclide quantification and Doppler echocardiography [flow (L/min) = mean velocity x area x ejection time x heart rate]. We found little difference between the magnitude of left-to-right shunt obtained with Doppler echocardiography and that with radioangioscintigraphy for a whole group of patients (N = 55, -11.42% to 12.04%) and for subgroups of ASD (n = 24, -12.49% to 12.19%) and VSD (n = 31, -10.69% to 12.23%). These results indicate that Doppler echocardiography, in comparison with radioangioscintigraphy, is sufficiently accurate for clinical estimation of the Q(p)/Q(s) ratio in isolated congenital heart disease with left-to-right shunt.