AbstractFe deficiency in early childhood is associated with long-term consequences for cognitive, motor and behavioural development; however explorations in healthy children from low risk, high-resource settings have been limited. We aimed to explore associations between Fe status and neurodevelopmental outcomes in low risk, healthy 2-year-olds. This study was a secondary analysis of a nested case–control subgroup from the prospective, maternal-infant Cork Babies after Screening for Pregnancy Endpoints: Evaluating the Longitudinal Impact using Neurological and Nutritional Endpoints (BASELINE) Birth Cohort Study. At 2 years, serum ferritin, Hb and mean corpuscular volume (MCV) were measured and neurodevelopment was assessed using the Bayley Scales of Infant and Toddler Development (n87). Five children had Fe deficiency (ferritin <12 µg/l) and no child had Fe deficiency anaemia (Hb<110 g/l+ferritin<12 µg/l). Children with microcytosis (MCV<74 fl,n13) had significantly lower mean cognitive composite scores (88·5 (sd13·3)v. 97·0 (sd7·8),P=0·04, Cohen’sdeffect size=0·8) than those without microcytosis. The ferritin concentration which best predicted microcytosis was calculated as 18·4 µg/l (AUC=0·87 (95% CI 0·75, 0·98),P<0·0001, sensitivity 92 %, specificity 75 %). Using 18·5 µg/l as a threshold, children with concentrations <18·5 µg/l had significantly lower mean cognitive composite scores (92·3 (sd10·5)v. 97·8 (sd8·1),P=0·012, Cohen’sdeffect size=0·6) compared with those with ferritin ≥18·5 µg/l. All associations were robust after adjustment for potential confounding factors. Despite a low prevalence of Fe deficiency using current diagnostic criteria in this healthy cohort, microcytosis was associated with lower cognitive outcomes at 2 years. This exploratory study emphasises the need for re-evaluation of the diagnostic criteria for Fe deficiency in young children, with further research in adequately powered studies warranted.