Platelet activation in preeclampsia is reflected by elevated levels of platelets exposing P-selectin. In plasma, a non-cell bound (soluble) form of P-selectin is present. Elevated levels of this soluble form have been reported in preeclampsia. Plasma P-selectin may consist of two fractions: microparticle (MP)--associated P-selectin and non-MP--associated P-selectin. In the present cross-sectional study, we investigated to which extent plasma P-selectin is MP--associated and whether such MP are elevated in preeclamptic patients. Preeclamptic patients (n=10) were matched with normotensive pregnant women (n=10) and non-pregnant controls (n=10). Plasma P-selectin was measured by ELISA. MP were isolated, double labelled with anti-CD61 (GPIIIa) and anti-CD62P (P-selectin) and subsequently analyzed with flowcytometry. Plasma P-selectin concentration was elevated in preeclamptic patients compared to non-pregnant controls (p=0.007), but not compared to normotensive pregnant women (p=0.210). Plasma P-selectin is partially MP--associated (3-5%). In pregnancy, the fraction of P-selectin exposing platelet-derived MP (PMP) (10.9%) was increased compared to non-pregnant controls (8%). This fraction further increased in preeclamptic patients (15.4%), and significantly differed from normotensive pregnant women (p=0.02). A minor fraction of plasma P-selectin is associated with PMP. The fraction of PMP exposing P-selectin is increased in preeclamptic patients and to a lesser extent in normotensive pregnancy. Because MP associated P-selectin exclusively originates from platelets, this fraction indicates platelet activation. Platelet activation is prominent in preeclampsia and this study proves that at least a part of the plasma P-selectin originates from platelets.