In 12 patients with ductus arteriosus a characteristic velocity pattern in the pulmonary artery was identified using pulsed Doppler echocardiography. This pattern, which was absent in 52 normal control subjects and 200 consecutive patients with cardiac lesions other than ductus, consisted of a pandiastolic velocity directed away from the transducer. This pattern is thought to represent the diastolic component of flow into the pulmonary tree which occurs as a result of the left to right shunt.