OBJECTIVE: To determine the minimal clinically important differences (MCIDs) of validated measures of systemic lupus erythematosus (SLE) disease activity in childhood-onset SLE. METHODS: Childhood-onset SLE patients (n = 98) were followed every 3 months for up to 7 visits (n = 623 total visits). Disease activity measures (European Consensus Lupus Activity Measure, Systemic Lupus Erythematosus Disease Activity Index, Systemic Lupus Activity Measure, British Isles Lupus Assessment Group, and Responder Index for Lupus Erythematosus [RIFLE]) were completed at the time of each visit. Physician-rated changes in the disease course (clinically relevant improvement, no change, clinically relevant worsening) between visits served as the criterion standard. RESULTS: MCIDs defined by mean change scores with improvement and worsening, or those based on the standard error of measurement with stable disease, were both small and did not discriminate well between disease courses (detection rates for improvement or worsening were all <55%). MCIDs based on discriminant and classification analyses yielded similar results. Alternative MCIDs, defined by a 70% predicted probability of improvement or worsening as per the discrimination analysis, were larger but underestimated the proportion of patients with change. The RIFLE only correctly identified 26% and 8% of episodes of clinically important worsening and improvement of childhood-onset SLE, respectively. CONCLUSION: The MCIDs of childhood-onset SLE disease activity measures are often small but similar to those reported for adults with SLE. Therefore, even small changes in disease activity scores can be clinically relevant. Low correct detection rates of these MCID thresholds for changes in disease course support the notion that worsening and improvement with childhood-onset SLE, or its response to therapy, is unlikely to be captured adequately by validated measures of disease activity alone.