In extremely preterm infants, the protective capacity for cerebral blood flow (CBF) autoregulation may be impaired or absent, which increases the risk for developing cerebral lesions. The purpose of this study was to quantify the simultaneous influence of several vital parameters, such as mean arterial blood pressure (MABP), PCO2, and PO2, on cerebral blood flow velocity (CBFv), which is used as a measure for CBF. In 16 mechanically ventilated infants of < 33 wk gestation, the CBFv in the internal carotid artery was measured every minute for 1 h by a computer-controlled pulsed Doppler device. MABP and transcutaneous PCO2 and PO2 were recorded as well. A multiple linear regression analysis was performed in each patient to determine the individual MABP, PCO2, and PO2 reactivities as a measure for CBF autoregulation. The medians (and ranges) of the whole group were an MABP reactivity of 7.5% (-12.5 to 20.1%) rise in CBFv/1 kPa rise in MABP, a PCO2 reactivity of 32.7% (-8.1 to 79.5%) rise in CBFv/1 kPa rise in PCO2, and a PO2 reactivity of -3.1% (-14.2 to 7.9%) fall in CBFv/1 kPa rise in PO2. In preterm infants, the individual's capacity for MABP-, PCO2-, and PO2-dependent CBF autoregulation can be estimated by means of the present method, even if the vital parameters change simultaneously.