Abstract Background Hip involvement predicts severe disease in juvenile idiopathic arthritis (JIA) and is accurately assessed by MRI. However, a child-specific hip MRI scoring system has not been validated. Objective To test the intra- and interobserver agreement of several MRI markers for active and chronic hip changes in children and young adults with JIA and to examine the precision of measurements commonly used for the assessment of growth abnormalities. Materials and methods Hip MRIs from 60 consecutive children, adolescents and young adults with JIA were scored independently by two sets of radiologists. One set scored the same MRIs twice. Features of active and chronic changes, growth abnormalities and secondary post-inflammatory changes were scored. We used kappa statistics to analyze inter- and intraobserver agreement for categorical variables and a Bland–Altman approach to test the precision of continuous variables. Results Among active changes, there was good intra- and interobserver agreement for grading overall inflammation (kappa 0.6–0.7). Synovial enhancement showed a good intraobserver agreement (kappa 0.7–0.8), while the interobserver agreement was moderate (kappa 0.4–0.5). Regarding acetabular erosions on a 0–3 scale, the intraobserver agreement was 0.6 for the right hip and 0.7 for the left hip, while the interobserver agreement was 0.6 for both hips. Measurements of joint space width, caput–collum–diaphyseal angle, femoral neck–head length, femoral width and trochanteric distance were imprecise. Conclusion We identified a set of MRI markers for active and chronic changes in JIA and suggest that the more robust markers be included in future studies addressing clinical validity and long-term patient outcomes.