Colour flow Doppler mapping can show abnormalities of flow even when the echocardiographic image is equivocal. Routine echocardiography in two patients who were thought to have pulmonary valve stenosis and a ventricular septal defect initially appeared to be normal. Colour flow Doppler mapping, however, showed multiple flow aliasing within the pulmonary artery, approximately 1.5 cm downstream from the pulmonary valve. More detailed examination by cross sectional echocardiography showed a membrane at this level in both patients. It is possible that more cases of this uncommon anomaly will come to light with increasing use of Doppler techniques.