Case 3 (MPO-ANCA) manifested as polyarticular arthritis without other organ involvement and was ultimately diagnosed as seronegative juvenile idiopathic arthritis (JIA), achieving complete remission with adalimumab therapy.<h4>Conclusions</h4>This case series highlights the diverse clinical and biological features of ANCA-positive conditions in children, emphasizing that ANCA positivity requires careful clinical correlation as it may indicate true vasculitis requiring aggressive treatment or alternative diagnoses such as JIA with incidental ANCA positivity. This evidence concerns the gene MPO and polyarticular arthritis.