Abstract. Objective: The aim of this prospective observational trial was to generate reference values of middle cerebral artery cerebral blood flow velocity (CBFv) in normotensive, ventilated newborn survivors born at 23–24 weeks’ gestation during the first week (with normal cranial ultrasounds during the hospitalization), and to analyze how reference values may be affected byPCO2. Methods: Baseline systolic, diastolic, and mean CBFv were determined for 12 infants born at 23–24 weeks’ gestation (birth weight 643±68 g) using transcranial Doppler ultrasound during several monitoring sessions during the first week, prior to intensive care procedures. Mean and 95% CI reference curves were generated using a locally weighted regression technique. Results: Mean CBFv increased from ∼10 to 21 cm/sec, and when baseline PCO2 was adjusted to 40, 50, and 60 mmHg, increased from ∼8.5 to 9, ∼12.5 to 16, and ∼16.5 to 23 cm/sec during the first week, respectively.Conclusion: Reference curves for middle cerebral artery CBFv for normotensive ventilated newborns born at 23–24 weeks’ gestation (with normal cranial ultrasounds) are presented for the first time. Since PCO2 is a potent regulator of CBF, when reporting cerebral hemodynamic values in the future, investigators must also report PCO2.