The peak amplitude of pulsatile cerebral electrical impedance (ΔZ) was compared with simultaneous133xenon clearance estimations of cerebral blood flow (CBF) on 28 occasions in nine infants receiving assisted ventilation who had changes in PaCO2and thereby presumably in cerebral blood flow. Percentage changes from one measurement to the next in each infant were compared. Using linear regression the relationship was ΔZ = 0.5 CBF – 1.5 withr= 0.67. The 95% confidence interval for the regression coefficient was 0.2–0.8 and the mean residual was 28%. Changes in cerebral blood flow in these clinical conditions were similarly detected by the two methods but ΔZ underestimated the magnitude of the change in comparison with CBF and its accuracy was insufficient to allow conclusions about the magnitude of small changes in cerebral blood flow in individual infants. (Pediatr Res24:461–464, 1998)