Cerebral blood flow was measured by intravenous 133Xe clearance one to four times during the first 48 h of life in 20 infants, born after 27 to 33 weeks of gestation, who were under mechanical ventilation and being monitored by amplitude integrated EEG, resulting in 36 sets of simultaneous CBF and EEG measurements. Mean CBF infinity, a measure of global flow to white as well as grey matter, was 10.0 ml/100 g/min +/- 3.5 SD. Definite EEG activity was seen with CBF infinity values as low as 5 ml/100 g/min. CBF infinity was related to the level of EEG activity, discontinuous EEG activity being associated with low CBF infinity (p = 0.014). It was not possible to determine if this relation indicated the presence of marginal ischaemia or primary inhibition of electrical activity resulting in decreased CBF, two infants developing periventricular leucomalacia, however, and one who developed intraparenchymatous hemorrhage were among the seven with CBF infinity values of less than or equal to 8 ml/100 g/min (p = 0.031). This suggests that ischaemia of periventricular white matter may have been present concurrently with cortical electrical activity.