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Background Adalimumab is an effective first-line biologic therapy used to treat children and young people (CYP) with juvenile idiopathic arthritis (JIA) and non-infectious uveitis (NIU). However, the development of neutralising antibodies may compromise adalimumab’s efficacy. This study investigated the incidence and clinical relevance of anti-adalimumab antibody (AAA) development in CYP treated with adalimumab for JIA and/or NIU.Methods A retrospective observational study was conducted within a specialist paediatric rheumatology service at a tertiary UK centre. CYPs receiving adalimumab between January 2015 and January 2024 who were tested for AAAs were included. Outcome measures included AAA positivity rate, serum drug levels, concurrent disease-modifying anti-rheumatic drug (DMARD) use, timing of antibody development and patient outcomes following AAA results.Results Among 390 CYP treated with adalimumab, 112 were tested for AAAs (156 total tests). Overall, 41.0% were AAA positive (>10 mg/L). All CYP with sub-therapeutic drug levels (<3 mg/L) were AAA positive. AAA positivity was higher in CYP not receiving concurrent DMARDs (52.4%) compared with those receiving full-dose methotrexate (21.1%) (OR 4.12, p=0.002). Among AAA-positive patients, 53.1% discontinued adalimumab treatment versus 15.2% of AAA-negative patients (OR 6.3, p<0.0001). Serial testing revealed fluctuating AAA levels, including reductions following corticosteroid use.Conclusions High AAA incidence was observed in CYP with JIA and NIU, with most tested during disease flare. AAA positivity correlated with sub-therapeutic drug levels and treatment discontinuation, particularly in the absence of concurrent full-dose methotrexate. Short-term variability in antibody titres suggests that a single positive result may not reliably indicate treatment failure.