Nineteen patients who had anatomical correction of transposition of the great arteries between the ages of one week and 10 years were investigated by pulsed and continuous wave Doppler echocardiography 1-98 months after operation. Peak blood flow velocities across the atrioventricular valves were significantly higher than normal, but no pathological regurgitation was seen. Systolic aortic velocities were normal in all but one patient, but mild aortic regurgitation was detected in ten. Increased flow velocities either at pulmonary valve level, at the pulmonary artery bifurcation, or in the left pulmonary artery were detected in 14 patients, the maximum calculated pressure drop being 36 mm Hg. Adequate Doppler sampling of the right pulmonary artery was not achieved in any of the cases, and sampling of the left pulmonary artery was possible in only ten. The principal limitation of the Doppler ultrasound method in this study was the inability to obtain flow velocities from the branch pulmonary arteries in most patients. Doppler echocardiography provides valuable information after the arterial switch operation and may reduce the need for repeat cardiac catheterisation.