Introduction: Mild iodine deficiency (MID) is a public health problem in Belgium, therefore, an agreement was signed in 2009 between the bakery sector and the Ministry of Health, to fortify bread with iodised salt. Median urinary iodine concentrations (UIC) in school-aged children is recommended for the evaluation of iodine status in the population although food habits of children may differ from those of adults.Objective: To evaluated iodine status of Belgian school children aged 6-12 years after the introduction of bread fortification with iodised salt. In addition, we investigated whether the median UIC in school-aged children was an adequate surrogate of iodine status of their mothers. Methods: We visited 60 schools selected from across the country in a van equipped with an ultrasound device. Thyroid volume of children was measured by ultrasound, a household salt sample and a urine sample, from the children and their mothers were collected. Results: Median UIC in children was 113.1µg/L and 84.4 µg/L among their mothers. The median UIC in school-aged children was lower in the south than in the north of Belgium (p< 0.001) and was higher in boys than in girls (p< 0.001). The percentage of children with goitre was 7.2%. Of the 904 salt samples received, 63.2% did not contain iodine. Conclusion: Voluntary fortification of bread with iodised salt corrects iodine deficiency in Belgian school-aged children but not in their mothers. Our findings suggest that the median UIC in school-aged children may not be an accurate surrogate of adults' iodine status. Therefore, monitoring iodine status should not be limited to school-aged children as currently recommended but should be extended to women of childbearing age.