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Objectives: To evaluate the European Consensus Lupus Activity Measurement (ECLAM) for responsiveness to change in disease activity when used in childhood-onset systemic lupus erythematosus (cSLE). To confirm the construct validity and to characterize the measurement properties of the ECLAM by assessing its ability to predict damage and steroid requirements., Methods: The disease courses of 66 newly diagnosed cSLE patients were reviewed. The ECLAM and Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) were scored for all clinic visits and hospitalizations. Damage was assessed at the end of the followup period using the Systemic Lupus International Collaboration Clinics/American College of Rheumatology Damage Index. Disease activity at the time of diagnosis, 6 months after diagnosis, at the time of first flare, and 6 months after first flare was used to estimate responsiveness of the measures. Responsiveness was measured by the effect size, the effect size index, the standardized response mean, and the relative efficiency index (REI). The measurement properties of the ECLAM and SLEDAI over time were examined by comparing the ability of both measures to predict damage and oral steroid requirement., Results: The ECLAM and SLEDAI are both responsive to change in disease activity irrespective of the statistic used. The ECLAM is more sensitive than the SLEDAI using the REI (all >1.9). Cumulative disease activity as measured by the SLEDAI or the ECLAM are important predictors of damage. There are no statistically important differences between the 2 measures with regard to their ability to predict steroid requirements., Conclusions: The ECLAM has construct validity in cSLE and, like the SLEDAI, is highly sensitive to clinically important change in disease activity. The ECLAM may be more responsive. The quantitative properties of the 2 measures are very similar. The SLEDAI likely remains the preferable disease activity measure for cSLE given its overall measurement properties and ease of use.