We hypothesized that conventional phototherapy (CPT) and fibre-optic phototherapy (FPT) could exert different effects on cerebral blood perfusion. Our aim was to assess this hypothesis in a prospective study of the cerebral haemodynamics in preterm infants. Twenty-three infants (gestational age <34 weeks) were randomized for CPT (n=12) and for FPT (n=11). Cerebral Doppler ultrasounds were performed on all infants immediately before phototherapy (time 0), 6–12 hours (time 1) and 24–36 hours (time 2) after the start of phototherapy, and 6–12 hours after discontinuing phototherapy (time 3). CPT and FPT were associated with a significant increase of peak-systolic blood flow velocity and mean blood flow velocity at time 1 and 2, which disappeared at time 3, whereas end-diastolic blood flow velocity and resistance index were unchanged. No difference was shown between the groups. CPT and FPT were found to exert similar effects on cerebral haemodynamics. Both were associated with an increase of cerebral blood flow velocity which ended when phototherapy was stopped.