The left ventricular ejection fraction was measured in 57 infants and children with congenital heart disease by both first-pass radionuclide angiography and left ventricular cineangiography. The correlation coefficient for the two sets of measurements was 0.79. In subgroups without left to right shunts (n = 21) and with left to right shunts (n = 36) it was 0.93 and 0.37, respectively. It is concluded that measurement of the left ventricular ejection fraction by the first-pass method is valid in children, provided there is no left-to-right shunt.