BACKGROUND: The role of circulating metabolites on child development is understudied. We investigated associations between children's serum metabolome and early childhood development (ECD). METHODS: Untargeted metabolomics was performed on serum samples of 5004 children aged 6-59 months, a subset of participants from the Brazilian National Survey on Child Nutrition (ENANI-2019). ECD was assessed using the Survey of Well-being of Young Children's milestones questionnaire. The graded response model was used to estimate developmental age. Developmental quotient (DQ) was calculated as the developmental age divided by chronological age. Partial least square regression selected metabolites with a variable importance projection ≥1. The interaction between significant metabolites and the child's age was tested. RESULTS: Twenty-eight top-ranked metabolites were included in linear regression models adjusted for the child's nutritional status, diet quality, and infant age. Cresol sulfate ( CONCLUSIONS: Serum biomarkers, including dietary and microbial-derived metabolites involved in the gut-brain axis, may potentially be used to track children at risk for developmental delays. FUNDING: Supported by the Brazilian Ministry of Health and the Brazilian National Research Council.