Background: Children in Flint, Michigan, have multiple risk factors for behavioural challenges, including exposure to lead during the Flint water crisis. However, their behavioural health status is largely unknown. Robust data from the Flint Registry can help understand the burden of behavioural outcomes and inform the allocation of resources. Objectives: This population-level evaluation of Flint children's behavioural outcomes aims to answer the question: What is the burden of parent-reported child behaviour problems in Flint Registry enrolled children? Methods: This cross-sectional study describes parent-reported behavioural outcomes of children 2-17 years old who enrolled in the Flint Registry between December 2018 and December 2020. Parents/guardians completed behavioural assessments including the Behavior Assessment System for Children (BASC-3) Parent Rating Scale and Behavior Rating Inventory of Executive Function (BRIEF2) Screening Parent Form. Demographics of enrolees were compared with census data. Composite BASC-3 T scores were compared with national norms. Distributions for clinically relevant categories of BASC-3 and BRIEF2 scores were examined across age and sex groups.Results: Of the 3579 children included in this study (mean age 9.73 ± 3.96 years), about half were female and 79.7% were eligible for free or reduced-price lunch. Almost half of the children were reported to have clinically concerning scores on the BASC-3 Parent Rating Scale (44.7%) and the BRIEF2 Screening Parent Form (46.7%). Across most age and sex groupings, the reported adaptive skills were relatively low and behaviour symptoms relatively high. Conclusions:Results reveal a substantial burden of parent-reported behavioural problems in Flint Registry children. This is clinically significant and indicates that a large number of children may require comprehensive neuropsychological evaluation and potential medical and/or educational services. Recognising the potential for longterm manifestations of childhood exposures to environmental hazards, longitudinal surveillance is critical to continue to identify and support participants.