Preeclampsia is pregnancy-specific syndrome, characterized by high blood pressure induced and proteinuria after 20 weeks of gestation. It complicates 2–۸% of all pregnancies and accounts for 25% of all maternal deaths and perinatal morbidity and mortality. Although preeclampsia is something more than simple gestational hypertension with proteinuria, development of proteinuria is still one significant and objective diagnostic measure of this disorder. Proteinuria is defined as excretion of more than 300 mg of protein in 24- hour urine collection, protein-creatinine ratio of 0.3 or higher in random urine samples, or consistent amount of protein (i.e., 30 mg per deciliter) in randomly taken samples of urine (i.e., +1 result on dipstick) [1]. Disorders of calcium metabolism, including hypocalciuria and low vitamin D level, have been consistently described, during in the course of pregnancy of women who later developed preeclampsia [2–۴]. Factors contributing to preeclampsia are diabetes, chronic hypertension before pregnancy, chronic kidney diseases, nulliparity, twin or multiple pregnancy, family history of preeclampsia or eclampsia, obesity, immune disorders and a personal history of preeclampsia, or eclampsia. The occurrence of preeclampsia in one pregnancy does not necessarily predict the occurrence of preeclampsia in subsequent pregnancies. However, its initial development is associated with a higher probability of it occurring in subsequent pregnancies. Vitamin D is especially important during pregnancy as low maternal vitamin D stores may contribute to problems such as low birth weight and small for gestational age infants, as well an increased risk of maternal comorbidities [5].Vitamin D deficiency is worldwide epidemic, with a prevalence that ranges from 18% to 84% depending on the country of residence, ethnicity, and local clothing customs and dietary intake [6, 7]. Clinical studies establishing an association between vitamin D levels and adverse pregnancy outcomes such as preeclampsia, gestational diabetes, and low birth weight, preterm labor, and caesarean delivery have conflicting results [8]. Previous studies have confirmed that low level of vitamin D disrupts the balance between Th1 and Th2 and contributes to overexpression of Th1 cytokines. The latter event affects immunological tolerance of embryo implantation. The studies suggest that deficiency of vitamin D could be associated with higher expression of Th1 which is observed in cases of preeclampsia [9]. There are different hypotheses concerning the etiology of preeclampsia, one of which is vitamin D deficiency in pregnancy. In the present study, vitamin D supplement was administered to pregnant women with history of preeclampsia in previous pregnancies. Considering the fact that one of the possible etiologies of preeclampsia is the increased requirement of vitamin D during pregnancy, that increased need is satisfied by prescribing vitamin D supplement and will allow us to examine its role in preventing preeclampsia.