Two-dimensional echocardiography and pulsed-Doppler studies have not proved to be reliable methods of assessing left-to-right shunt size in atrial septal defect. Doppler color-flow imaging displays the transatrial jet, providing a new dimension with the potential capability of quantifying left-to-right shunt size. Twenty-three patients with atrial septal defect were studied by color-flow imaging and cardiac catheterization. The defect size measured by two-dimensional echocardiography, the maximal color-flow jet width in the atrial septum, and the maximal color-flow jet area in the right atrium were correlated with cardiac catheterization-derived left-to-right shunt size. Correlation coefficients were 0.57 (p less than 0.01), 0.67 (p less than 0.001), and 0.65 (p less than 0.01), respectively. Atrial septal color-flow jet width distinguished patients with less than a 2:1 left-to-right shunt size ratio (eight patients, jet width less than 15 mm in in all) from patients with greater than a 2:1 left-to-right shunt size ratio (15 patients, jet width greater than 15 mm in all). These results indicate that Doppler color-flow imaging can distinguish left-to-right shunt size in atrial septal defect accurately enough to influence decisions with regard to subsequent patient management.