Systemic-onset juvenile idiopathic arthritis is an inflammatory disease of unknown cause and is not commonly associated with kidney involvement. We describe 3 patients with systemic-onset juvenile idiopathic arthritis with high disease activity who developed antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis 1-6 years after the onset of systemic-onset juvenile idiopathic arthritis. Renal and systemic-onset juvenile idiopathic arthritis remission occurred in one patient under anti–interleukin 1 (anti–IL-1) treatment associated with immunosuppressive drugs. The other 2 patients developed end-stage renal disease, and one of those patients died. This report suggests that the diagnosis of ANCA-associated glomerulonephritis must be considered in patients with systemic-onset juvenile idiopathic arthritis with persistently active systemic disease who present with proteinuria. Furthermore, use of an anti–IL-1 agent might be an effective therapeutic option.