Background: Preeclampsia affects about 7- 10% of pregnant women. It is a multisystem disorder characterized by new onset hypertension (>140/ 90 mmHg) with coexisting proteinuria (>300 mg/24 h) and other maternal organ dysfunction. Regulatory T cells act as potent suppressors of inflammation that prevent activation of maternal T cells against fetal cells. IL‐10 plays a vital role in the reduction of inflammation‐mediated vascular dysfunction, hypertension, and hypoxia during gestation. Objectives: The study aims to assess the frequency of T regulatory cells and IL-10 in peripheral blood from PE patients and normal pregnant women during the third trimester of pregnancy. Methodology: This study was carried out on 25 pregnant women with PE at 30–40 weeks of gestation selected from the pregnant women hospitalized at El-Hussein Hospital of Al-Azhar University and Saied Galal hospital and 25 healthy gestational‐ matched pregnant women as a control group. Flow cytometric analysis of Tregs was carried out by the following antibodies: phycoerythrin-cy7-conjugated anti-CD127, phyco-erythrin-conjugated CD25 and fluorescein isothiocyanate-conjugated anti-CD4. The sandwich kit ELISA is for the accurate quantitative detection of Human IL-10 in serum. Results: A highly significant difference (p<0.001**) was found as regard T regulatory cells% between preeclampsia patients and healthy control groups. As regard IL-10 serum level, there were significant differences (p<0.001**) between the two studied groups. Conclusions: These data suggested that Treg cells and IL-10 serum level are extremely critical determinants of safe pregnancy outcome.