Objectives: Comparison of fetal umbilical and middle cerebral artery flow between early- and late-onset preeclampsia. Material and methods: Our study was conducted among 50 patients with preeclampsia in the third trimester of pregnancy and included 30 women with early-onset and 20 with late-onset disease. Early-onset preeclampsia (EOP) and late-onset preeclampsia (LOP) were defined as onset of the disease before and after 34 weeks of gestation, respectively. Doppler examinations of the fetal umbilical and middle cerebral artery were performed in all patients. Pulsatility Index (PI) and cerebral-umbilical ratio (CUR) were measured each time. Results: Mean value of the umbilical artery PI was significantly higher in fetuses of patients with EOP in comparison to LOP, whereas mean PI value in MCA was significantly lower in fetuses from the group with EOP than LOP. The percentage of abnormal results of fetal Doppler examinations, both in the umbilical artery and middle cerebral artery, was significantly higher in EOP than in LOP. The same tendency was observed for CUR. Conclusions: 1. Early-onset preeclampsia is characterized by significantly higher degree of placental insufficiency than late-onset disease. 2. The obtained results indicate a significant, pathological role of the placenta in early-onset preeclampsia. 3. Pathophysiological differences between early- and late-onset preeclampsia lead to different clinical approach to patients, depending on the type of the disease, including emphasis on Doppler examination in the early-onset preeclampsia. 4. The presence or absence of placental insufficiency in pregnancy complicated by preeclampsia seems to determine the clinical course of the disease, thus allowing for an alternative classification of the condition into placental and maternal preeclampsia.