OBJECTIVE To evaluate the effects of fortified drinking water, with different concentrations of iron added, on hemoglobin and hematocrit values in preschoolers. METHODS Double-blind, randomized cluster clinical trial, with children aged 2 to 5 years of age, from 4 state-run schools, forming 1 group for each school. For fortification, ferrous sulphate in concentrations of 5 mg of elemental iron per liter of water (group A), 7.5 mg (group B), and 10 mg (group C), was used during a period of 4 months. In group D, the control, a placebo (Bixa orellana) was added. Hemoglobin and hematocrit values were checked before and after intervention. RESULTS Before fortification, hemoglobin and hematocrit averages were below the reference values adopted in all groups. After fortification, the prevalence of anemia showed a reduction in the 4 groups, which was more pronounced in group B, at 48.3%. The hemoglobin values in groups B (11.5) and C (11.4) were statistically similar. However, the average consumption of water/day/student was lower in group C. Comparison of hemoglobin values between groups A (11.2) and D (11.0) did not show a significant difference, suggesting insignificant efficacy with 5 mg Fe/L fortification. CONCLUSIONS The consumption of drinking water fortified with 7.5 mg of elemental iron/L water resulted in greater adhesion and an increase in hemoglobin values, with a reduction in the prevalence of anemia.