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Background: During pregnancy, changes in blood coagulation and fibrinolysis create a hypercoagulable state. In the puerperium this thrombogenicity is even higher, and the chance of developing thromboembolism is 3-5 times higher in this period than during pregnancy. In preeclampsia, platelets are activated and play a substantial role in the pathogenesis of the disease. Systematic information on longitudinal changes in platelet number and size postpartum after normotensive and preeclamptic pregnancies is not available., Methods: We measured platelet number, mean platelet volume and the median volume of the 20% largest platelets in eleven preeclamptic and eleven normotensive pregnant women matched for mode of delivery. The blood samples were taken antepartum and every 2-3 days in the postpartum period until the platelet count decreased/normalized., Results: In the preeclamptic group, the platelet count increased significantly from 240x10(9)/l antepartum to 621x10(9)/l on day 6-14 postpartum (p<0.01). In the control group, the platelet count increased from 214x10(9)/l antepartum to 251x10(9)/l on day 2-5 (p<0.01) and 351x10(9)/l on day 6-14 postpartum (p<0.01). The platelet count was significantly higher in the preeclamptic than in the control group 6-14 days postpartum (p<0.01). Antepartum, mean platelet volume and the median of the 20% largest platelets were significantly higher in the preeclamptic than in the control group., Conclusion: The platelet count is significantly increased postpartum both after normotensive, and 2-3 fold more after preeclamptic pregnancies. The time to peak values is between 6-14 days, usually at a time when patients are discharged from hospital.