The purpose of this study was to evaluate the distinct pathogenic mechanisms underlying chronic hypertension in pregnancy and preeclampsia in terms of oxidative stress and vascular reactivity. A total of 17 women with uncomplicated pregnancies, 30 women with preeclampsia and 17 women with chronic hypertension were evaluated. We measured serum derivatives of reactive oxygen metabolites (d-ROMs; marker of oxygen free radicals), flow-mediated vasodilation (FMD; marker of endothelial function) and intima-media thickness in the carotid artery (IMT; marker of atherogenesis) during pregnancy and 1 month after delivery. Serum d-ROM concentrations were significantly higher in women with chronic hypertension and severe preeclampsia than in the control group during pregnancy. d-ROM concentrations in all groups significantly decreased to similar levels 1 month after delivery. FMD was significantly lower during pregnancy in preeclamptic and chronic hypertension groups compared with the control group. FMD in preeclamptic groups significantly increased and normalized to control levels after delivery. Similarly, FMD in the chronic hypertension group significantly increased after delivery but was still lower. IMT in the chronic hypertension group was significantly higher than that in control and preeclamptic groups. These findings suggest that endothelial dysfunction induced by enhanced oxidative stress is reversible in women with preeclampsia, whereas impaired vascular reactivity may be associated with atherosclerotic changes in women with chronic hypertension.