OBJECTIVE: To test the hypothesis that preeclampsia is associated with impaired dynamic cerebral autoregulation. METHODS: In a prospective cohort analysis, cerebral blood flow velocity of the middle cerebral artery (determined by transcranial Doppler), blood pressure (determined by noninvasive arterial volume clamping), and end-tidal carbon dioxide were simultaneously collected during a 7-minute period of rest. The autoregulation index was calculated. Values of 0 and 9 indicated absent and perfect autoregulation, respectively. Student t test was used, with P<.05 considered significant. RESULTS: Women with preeclampsia (before treatment, n=20) and their normotensive counterparts (n=20) did not differ with respect to baseline characteristics, except for earlier gestational age at delivery (36 3/7 [24 4/7-40 2/7] compared with 39 2/7 [32 0/7-41 0/7]; P<.001) and higher blood pressure in women with preeclampsia. Autoregulation index was significantly reduced in preeclamptic women compared with normotensive women in the control group (5.5 +/- 1.7 compared with 6.7 +/- 0.6; P=.004). CONCLUSION: Women with preeclampsia have impaired dynamic cerebral autoregulation. The fact that blood pressure does not correlate with autoregulation functionality may explain why cerebral complications such as eclampsia can occur without sudden or excessive elevation in blood pressure.