Objective: To study the effect of calcium supplementation on reduction of pre-eclampsia in Iranian women at high risk of pre-eclampsia. Methods: Thirty pregnant women at high risk of developing pre-eclampsia were randomized to 2 g of daily calcium and placebo. Subjects and investigators were blinded to treatment allocation. The inclusion criteria were positive rollover test, having at least one risk factor for prl-eclampsia. between 28 and 32 weeks of pregnancy, and blood pressure less than 140/90 (mmHg). Exclusion criteria were having any chronic condition. Pre-eclampsia was defined as systolic/diastolic blood pressure over 140/90 plus proteinuria, All the subjects were followed up to delivery. Result: A sevenfold reduction in the occurrence of pre-eclamsia were seen among the calcium group compared with the placebo group (7 out of 15 developed pre-eclampsia in control group vs, 1 out of 15 in calcium group, P = 0.014). No major effect on pregnancy induced hypertension was seen (66% in placebo and 76% in calcium group developed hypertension) but the onset of hypertension was delayed 3 weeks in the calcium group. A Longer duration of pregnancy was seen in calcium than placebo (mean +/- S.D. = 37 +/- 2 for calcium and 34 +/- 2 for placebo, P < 0,05). Infants born to the calcium group, on average, were 552 g heavier than infants born to the placebo group, P < 0.05, Conclusion: Calcium supplementation is beneficial for preventing pre-eclampsia among Iranian women at high risk of developing pre-eclampsia. (C) 2001 International Federation of Gynecology and Obstetrics. All rights reserved.