Elevated plasma endothelial microparticle levels have been found to be elevated in women with preeclampsia. However, their role in distinguishing preeclampsia from gestational hypertension remains to be elucidated. The objectives of this study were to compare endothelial microparticle levels among patients with preeclampsia, gestational hypertension, and healthy pregnant control subjects and to evaluate the effect of plasma from women with preeclampsia and gestational hypertension on the release of endothelial microparticles by renal microvascular endothelial cells. A prospective study was conducted on 52 women with preeclampsia, 20 women with gestational hypertension, and 38 healthy pregnant control subjects. Endothelial microparticles were measured by flow cytometry with fluorescent monoclonal mouse anti human antibodies against CD31, CD42b, and CD62E.CD31+ /42b endothelial microparticle levels were 10497 ± .5145 counts/μ L in women with preeclampsia versus 6768 ± .1810 counts/μ L in women with gestational hypertension (P <. 01). In control subjects, CD31+ /42b- endothelial microparticle levels were 6119 ± 3592 counts/μ L. CD62E+ endothelial microparticle levels were1930 ± .966 counts/μ L in women with preeclampsia versus 822 ± .150 counts/μ L in women with gestational hypertension (P <.01). In control subjects, CD62E+ endothelial microparticle levels were 712 ± .160 counts/μ L. Incubation of renal microvascular endothelial cells with plasma from women with preeclampsia resulted in a rise in CD31+ and CD62E+ endothelial microparticle levels as compared with women with gestational hypertension and control subjects. Endothelial microparticle levels are higher in women with preeclampsia than in women with gestational hypertension and control subjects. The measurement of endothelial microparticles may be useful as a diagnostic tool for preeclampsia in pregnant women. Elevated plasma endothelial microparticle levels have been found to be elevated in women with preeclampsia. However, their role in distinguishing preeclampsia from gestational hypertension remains to be elucidated. The objectives of this study were to compare endothelial microparticle levels among patients with preeclampsia, gestational hypertension, and healthy pregnant control subjects and to evaluate the effect of plasma from women with preeclampsia and gestational hypertension on the release of endothelial microparticles by renal microvascular endothelial cells. A prospective study was conducted on 52 women with preeclampsia, 20 women with gestational hypertension, and 38 healthy pregnant control subjects. Endothelial microparticles were measured by flow cytometry with fluorescent monoclonal mouse anti human antibodies against CD31, CD42b, and CD62E. CD31 + / 42b endothelial microparticle levels were 10497 ± .5145 counts / μL in women with preeclampsia versus 6768 ±. 1810 count s / μL in women with gestational hypertension (P <. 01). In control subjects, CD31 + / 42b-endothelial microparticle levels were 6119 ± 3592 counts / μL. CD62E + endothelial microparticle levels were 1930 ± .966 counts / μL in women with preeclampsia versus 822 ± .150 counts / μL in women with gestational hypertension (P <.01). In control subjects, CD62E + endothelial microparticle levels were 712 ± .160 counts / μL Incubation of renal microvascular endothelial cells with plasma from women with preeclampsia resulted in a rise in CD31 + and CD62E + endothelial microparticle levels as compared with women with gestational hypertension and control subjects. Endothelial microparticle levels are higher in women with preeclampsia than in women with gestational hypertension and control subjects. The measurement of endothelial microparticles may be useful as a diagnostic tool for preeclampsia in pregnant women.