Juvenile idiopathic arthritis (JIA) is a common pediatric rheumatic disease. Renal manifestations have been rarely observed in JIA, although amyloidosis could be a renal complication in systemic JIA (sJIA). To investigate renal damage in JIA children and to establish the relationship with treatment. Blood urea nitrogen (BUN), creatinine, cystatin C (CysC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), urinary albumin excretion (UAE), estimated glomerular filtration rate (eGFR), and renal resistive index (RRI) were assessed in 49 JIA children (9 boys/40 girls, mean age 10.3 ± 3.8 years) and in 49 healthy controls (24 boys/25 girls, mean age 11.3 ± 3.4 years). Twenty-two JIA patients were on methotrexate (MTX) therapy (group A) and 27 on biologic drugs (group B). CysC and BUN (respectively, 0.8 ± 0.1 vs. 0.7 ± 0.1 mg/dl; 13.3 ± 2.9 vs. 11.7 ± 1.4 mg/dl) were higher (p ≤ 0.001) whereas creatinine and eGFR (respectively, 0.5 ± 0.1 vs. 0.6 ± 0.1 mg/dl; 99.2 ± 10.5 vs. 122.5 ± 19.8 ml/min/1.73 m2) were lower in JIA children as compared to controls (p < 0.001). UAE resulted higher in patients than in controls (p = 0.003). Mean RRI was higher in JIA children than controls (0.7 ± 0.04 vs. 0.6 ± 0.04; p < 0.001). Group B showed higher mean RRI than group A (0.7 ± 0.1 vs. 0.7 ± 0.04; p < 0.001). Associations were found between RRI and ESR, JADAS-27, disease state, BMI-SDS (p < 0.001), CRP (p = 0.003) and eGFR (p = 0.001). JIA children had reduced eGFR, increased UAE and higher RRI values, than controls. RRIs were higher in patients on biologic drugs than MTX group and were associated with inflammation indexes and disease state, suggesting a direct effect of the disease
