© 2019, American College of Rheumatology Objective: To develop a Childhood Lupus Improvement Index (CHILI) as a tool to measure response to therapy in childhood-onset systemic lupus erythematosus (cSLE), with a focus on clinically relevant improvement (CRI cSLE ). Methods: Pediatric nephrology and rheumatology subspecialists (n = 213) experienced in cSLE management were invited to define CRI cSLE and rate a total of 433 unique patient profiles for the presence/absence of CRI cSLE . Patient profiles included the following cSLE core response variables (CRVs): global assessment of patient well-being (patient-global), physician assessment of cSLE activity (MD-global), disease activity index score (here, we used the Systemic Lupus Erythematosus Disease Activity Index), urine protein-to-creatinine ratio, and Child Health Questionnaire physical summary score. Percentage and absolute changes in these cSLE-CRVs (baseline versus follow-up) were considered in order to develop candidate algorithms and validate their performance (sensitivity, specificity, area under the receiver operating characteristic curve [AUC]; range 0–1). Results: During an international consensus conference, unanimous agreement on a definition of CRI cSLE was achieved; cSLE experts (n = 13) concurred (100%) that the preferred CHILI algorithm considers absolute changes in the cSLE-CRVs. After transformation to a range of 0–100, a CHILI score of ≥54 had outstanding accuracy for identifying CRI cSLE (AUC 0.93, sensitivity 81.1%, and specificity 84.2%). CHILI scores also reflect minor, moderate, and major improvement for values exceeding 15, 68, and 92, respectively (all AUC ≥0.92, sensitivity ≥93.1%, and specificity ≥73.4%). Conclusion: The CHILI is a new, seemingly highly accurate index for measuring CRI in cSLE over time. This index is useful to categorize the degree of response to therapy in children and adolescents with cSLE.