It has been suggested that cerebral oximetry can detect acute and chronic changes in cerebral oxygen saturation due to pregnancy related complications. Furthermore, regional cerebral oxygenation saturation (rcSO(2)) decreases were obtained during spinal anesthesia for cesarean section. The aim of this prospective observational study is to compare the effects of spinal anesthesia on rcSO(2) in preeclamptic and normotensive pregnant women. Preeclamptic (Group P, n = 24) and normotensive (Group N, n = 25) women with gestational week 32 and above, and scheduled for cesarean section under spinal anesthesia were included in this study. In addition to routine monitoring, rcSO(2) values obtained with right and left frontal cerebral sensors (rcSO(2)right and rcSO(2)left) were recorded before (baseline) spinal injection and during the surgery. The baseline rcSO(2) values were similar in both groups. In Group P, rcSO(2)left values were higher than Group N only 3 and 5 min after spinal injection. In Group N, rcSO(2) values decreased from baseline 1, 3, 5, and 10 min after spinal injection. In Group P, rcSO(2) values decreased from baseline 1, 3, 5, 10, 30 and 35 min after spinal injection and at the end of the operation. There was no difference between the number of declines and the patients with rScO(2) below the cerebral hypoxic threshold. There was a positive correlation between rcSO(2) and blood pressure only 5 min after spinal injection, but no correlation with peripheral oxygen saturation was detected during the operation. There is decrease in rcSO(2) values after spinal anesthesia correlating with hypotension in preeclamptic women. However, the decrease is less than that of normotensive pregnant women, especially the first 5 min after spinal injection when the blood pressure is lowest. The clinical impact of these results and the relationship between cerebral desaturation and neurological complications remain to be determined.