OBJECTIVE: To evaluate effects of high and low dose folic acid on levels of hemocysteine (Hcy) concentration during first trimester of pregnancy and at delivery, and to examine the association of Hcy serum levels and preeclampsia. METHODS: A single blinded randomized clinical trial was conducted in Tabriz, Iran, from 2006-2013, in 400 nulliparous pregnant women in 2 groups, who received folic acid daily from early pregnancy until delivery (5 mg/day and 0.5 mg/ day respectively). The incidence of pregnancy induced hypertension, preeclampsia, eclampsia and laboratory changes in the levels of serum Hcy, Platelet numbers, lactate dehydrogenase, uric acid, serum and urine creatinine and serum and urine protein were compared. Maternal blood pressure and weight were checked in each visit including labor time. The number of abortions, premature labor, IUGR, fetal death, PIH, preeclampsia, eclampsia , birth weight, congenital abnormalities, date of pregnancy termination, and meconium staining were reported in a questionnaire. Healthy mothers with a singleton pregnancy aged between 20-30 years old were included and patients with heart disease; chronic hypertension; diabetes mellitus; collagen vascular diseases; chronic renal disease; and who taking medications other than calcium and ferrous sulfate and vitamin B6 were excluded