Cerebrovascular autoregulation is the mechanism by which changes in cerebral blood flow are prevented during fluctuations in mean arterial blood pressure. Doppler ultrasound measurement of cerebral blood flow velocity provides a reliable indirect technique to estimate cerebral blood flow. In 48 stable preterm infants less than or equal to 32 weeks gestation, we studied the mean flow velocity in the pericallosal artery at 12, or at 12 and 72 h of age with two-dimensional/pulsed Doppler ultrasound and correlated the mean flow velocity with the simultaneously obtained mean arterial blood pressure values. Mean flow velocity was stable at a mean arterial blood pressure ranging from 31 to 40 mm Hg, but changed proportionally with mean arterial blood pressure values outside this narrow range. Multiple regression analysis showed that mean flow velocity was primarily determined by mean arterial blood pressure. These data suggest that in preterm infants regulation of cerebral blood flow velocity occurs only over a narrow range of mean arterial blood pressure values.