OBJECTIVE: To prospectively validate the provisional criteria for the evaluation of response to therapy in children with systemic lupus erythematosus (SLE). METHODS: In this multicenter study, childhood-onset SLE patients (n = 98; 81 girls, 17 boys, 50% white, 88% non-Hispanic) were followed every 3 months for up to 7 visits (total number of visits 623). The 5 childhood-onset SLE core response variables were obtained at the time of each visit: 1) physician assessment of overall disease activity, 2) parent assessment of patient well-being, 3) Child Health Questionnaire, 4) proteinuria, and 5) global disease activity measure score, as measured by the European Consensus Lupus Activity Measure (ECLAM), the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI), or the Systemic Lupus Activity Measure (SLAM). Physician-rated relevant changes in the disease course (clinically relevant improvement, no change in disease, or worsening) between visits served as the criterion standard. Mixed models were used to assess the diagnostic accuracy of the 4 highest-ranked provisional definitions of response to therapy. RESULTS: There were 89 episodes of clinically relevant improvement between 2 consecutive visits, and 448 episodes without improvement. Irrespective of the choice of the global disease activity measure (ECLAM, SLAM, SLEDAI), sensitivities of all 4 highest-ranked definitions were low (all < or =31%), whereas their specificities were excellent (all >88%). Using logistic models, alternative definitions can be developed with both 80% sensitivity and specificity. CONCLUSION: The provisional criteria of response to therapy in childhood-onset SLE may have considerably lower sensitivity than previously reported. Additional validation in clinical trials is necessary to further evaluate the measurement properties of the provisional Paediatric Rheumatology International Trials Organisation/American College of Rheumatology criteria for response to therapy in children with SLE.