This study aims to evaluate a prototype device detecting zinc protoporphyrin-IX fluorescence non-invasively from the intact oral mucosa in children. The prototype device has shown high sensitivity and specificity in women after delivery for iron deficiency. Children are at increased risk for iron deficiency and prevention methods are not established jet. Zinc protoporphyrin-IX is an early indicator of iron deficiency and may be more sensitive than other established parameters. The prototype device is used to measure the erythrocyte zinc protoporphyrin-IX/heme ratio in children aged 9 months to 5 years. Children in this age are at increased risk for iron deficiency as they are growing rapidly and iron deficiency in this age may affect the neurodevelopment and immune system adversely. It is proposed that these effects cannot be rectified by iron supplementation in later years. The results from the non-invasive measurements are compared to reference measurements of the erythrocyte zinc protoporphyrin-IX/heme ratio from residual blood samples from the same patients and to other indicators of iron status, including hemoglobin, ferritin, serum iron, transferrin, transferrin saturation and soluble transferrin receptor.