Preeclampsia is the most common disease of pregnancy, occurring in up to 10% of the pregnant population. The cause of the disease is as yet undetermined; however, most of the clinical effects are commonly attributed to damage to the endothelial layer, leading to increased pressor activity of all the maternal blood vessels. Therefore, we suspected that if obstructive sleep apnea (OSA) coexisted with preeclampsia in pregnancy, the hemodynamic effects of the OSA would be markedly potentiated. To test this hypothesis, we performed full sleep studies and overnight beat-to-beat blood pressure (BP) monitoring. The control patient group included 10 pregnant women with OSA and no evidence of hypertensive disease either before or during their current pregnancy. The test group included 10 women with preeclampsia and coexisting OSA. The pressor responses to obstructive respiratory events during sleep were enhanced in preeclamptic patients compared with control OSA patients (21+/-2/12+/-1 mm Hg and 38+/-5/25+/-4 mm Hg above baseline in control OSA and preeclamptic OSA patients, respectively, P = .005/.005). In contrast, there was no difference in heart rate responses between the two groups of subjects (34+/-5 beats/min and 49+/-13 beats/min above baseline in control and preeclamptic patient groups, respectively, P = .326). We suggest that the augmented pressor responses in preeclamptic women occur as a result of maternal endothelial damage induced by the preeclampsia disease process. These findings may have important implications in the management of preeclamptic patients.