Cerebral blood flow velocity was studied with two-dimensional/pulsed Doppler ultrasound before, during and after discontinuation of phototherapy in 22 preterm infants (gestational age < or =32 weeks), who were treated for a minimum of 12h with blue-light phototherapy for non-haemolytic hyperbilirubinaemia. Before the cerebral blood flow velocity measurements, patency of the ductus arteriosus was diagnosed by Doppler echocardiography. All infants had normal brain ultrasound scans. Mean cerebral blood flow velocity increased significantly after initiation of phototherapy in all infants. Only in "healthy" (non-ventilated) infants did cerebral blood flow velocity return to pre-phototherapy values (baseline) after discontinuation of phototherapy, whereas in "unhealthy" (ventilated) infants cerebral blood flow velocity did not return to baseline. In 10 infants the ductus arteriosus reopened during phototherapy. In those infants, mean cerebral blood flow velocity returned to pre-phototherapy values after 2 h of phototherapy prior to its discontinuation.