Introduction: Iron deficiency anaemia is a serious public health problem in developing countries, especially among children, as it is associated with serious developmental problems. Objective: To assess the effects of weekly ferrous sulphate supplementation on haemoglobin (Hb) levels and the prevalence of anaemia in children aged 2 to 3 years. Methods: A cluster-randomized clinical trial was conducted; two schools were randomly chosen. In the first school, the children received 6mg/kg of elemental iron in the form of iron sulphate once a week (intervention). In the other school, the children received a placebo (control). The intervention group had 44 participants at the end of the study, and the control group had 48 children. Blood samples were taken at baseline and at the end of the study to assess serum Hb levels and anaemia prevalence. The intervention lasted 14 weeks. Results: There was a mean increase in Hb of 0.85g/dL (p=0.0003) in the intervention group and a decrease of 0.74g/dL (p=0.0001) in the control group. The prevalence of anaemia significantly decreased in the weekly supplementation group with p=0.0002. Conclusion: Weekly iron supplementation in preschool-age children promoted a significant increase in Hb levels and a decrease in the prevalence of anaemia.