Blood flow in the pulmonary artery was studied by Doppler colour flow mapping and cardiac catheterisation in 19 patients with a ductus arteriosus and different pulmonary artery pressures. In the four patients with normal pulmonary artery pressures colour Doppler flow mapping showed multicoloured wide and long systolic and diastolic jets in the pulmonary artery. In the 15 patients with raised pulmonary arterial pressure the systolic jets varied from multicoloured to red and were thinner: in patients with considerably raised pulmonary arterial pressure the jets became redder during diastole. The Doppler velocity tracings showed that in patients with normal pulmonary artery pressures the mean peak systolic velocity was higher than the mean end diastolic velocity--so that in all four the ratio of peak systolic velocity to end diastolic velocity was less than 2. The mean peak systolic velocity was much higher than the mean end diastolic velocity in 13 of the 15 patients with raised pulmonary artery pressure; this meant that the ratio of peak systolic velocity to end diastolic velocity was greater than 2 in 10 of 11 patients. The end diastolic velocity was significantly lower in those patients with raised pulmonary artery pressure than in those with normal artery pressure. There was an inverse linear correlation between the mean pulmonary artery pressure and end diastolic ductal jet velocity in 17 of the 19 patients. Colour flow mapping and this quantitative Doppler technique can detect pulmonary artery hypertension in patients with a ductus arteriosus.