In preterm infants, persistent ductus arteriosus (PDA) fails to close soon after birth and becomes symptomatic (sPDA) in about 40% of the infants, causing cardio-respiratory deterioration by a left-to-right shunt across the PDA. Aortic run-off of blood, predominantly occurring during ventricular diastole, causes an abnormal diastolic retrograde aortic blood flow. This aortic reverse flow can be assessed semi-quantitatively in a noninvasive way, using continuous-wave Doppler-ultrasonography. An increased ratio (R/F ratio) of the abnormal retrograde aortic blood flow (R) related to the normal forward flow (F) in the aorta indicates presence of sPDA in preterm infants. The R/F ratio was assessed in 30 premature infants, including 13 cases without sPDA, and 17 infants with sPDA--in 12 of them before and after surgical ligation of PDA, in five concomitantly to pharmacological closure of PDA by the application of indomethacin. The R/F ratio was low in all infants without sPDA and in infants following surgical ligation of PDA. On the other hand, a high R/F ratio was found in all patients with sPDA before specific treatment. During indomethacin-induced closure of PDA the R/F ratio decreased continuously, whereas it remained high in infants with sPDA not responding to indomethacin treatment.