Serial measurements of cerebral blood flow velocity (CBFV) were made in 29 preterm infants, using continuous wave Doppler ultrasound. CBFV was measured in both anterior cerebral arteries and quantitative measurements of CBFV were determined using the area under the velocity curve. In all ventilated infants, CBFV increased significantly during the first 6 hours of life and continued to increase until 16 hours of age. Thereafter, CBFV remained relatively constant. This increase in CBFV was primarily the result of increased diastolic flow. Three infants who had evidence of intraventricular haemorrhage on cranial ultrasound, had similar CBFV compared with the infants with no evidence of haemorrhage. Two infants died and both demonstrated areas of periventricular leukomalacia at autopsy. These infants had a prolonged period of low CBFV. These measurements provide normal data for ventilated, preterm infants. As previously suggested in term infants, the initial rise in CBFV may be secondary to closure of the ductus although a generalized decrease in peripheral vascular resistance could also be a contributing factor. Fluctuations in CBFV rather than individual readings are probably more important in the genesis of IVH. An episode of significantly reduced CBFV is a poor prognostic sign.