OBJECTIVE: To study the effect of iron supplementation on pregnancy outcome in pregnant women with haemoglobin (Hb) > or = 13.2 g/dl. DESIGN: A randomised, double-blind, placebo-controlled trial. SETTING: Routine health services. POPULATION: Seven hundred and twenty-seven pregnant women with Hb > or = 13.2 g/dl in the early stage of the second trimester. METHODS: Each woman took one ferrous sulphate [DOSAGE ERROR CORRECTED] tablet (150 g tablet, containing 50 mg of elemental iron) [DOSAGE ERROR CORRECTED] daily in the case group (n = 370) or placebo in the control group (n = 357) throughout pregnancy. MAIN OUTCOME MEASURES: Pregnancy outcome. RESULTS: While there were no significant differences in demographic and obstetric characteristics between the two groups before any intervention, small-for-gestational-age birth rate and the number of women with hypertension disorder increased significantly in the case group in comparison with the control group (57 [15.7%] versus 36 [10.3%], P = 0.035, 10 [2.7%] versus 3 [8%], P = 0.05, respectively). CONCLUSIONS: Our finding proves that routine iron supplementation in nonanaemic women is not rational and may be harmful.