Several clinical measures of disease activity in patients with systemic lupus erythematosus (SLE) have been proposed, but it is unclear which of these most accurately reflects SLE activity and which is most sensitive to change. The degree of change in SLE activity that patients consider important is also unknown. This prospective longitudinal study will determine the relative accuracy and relative sensitivity to change of five SLE activity measures (the Systemic Lupus Activity Measure, the Systemic Lupus Erythematosus Disease Activity Group index, and the European Consensus Lupus Activity Measurement), and will determine the importance to patients of changes in SLE activity. Sixty patients with active SLE will be examined every two weeks for 40 weeks. At each assessment, SLE activity will be scored using each of the five SLE activity measures, physician global assessments of SLE activity will be recorded, and patients will judge the importance of interval changes in SLE activity. The accuracy of the SLE activity measures will be evaluated by comparing the extent to which changes in different measures correlate with each other over time (i.e. longitudinal construct validity). Sensitivity to change will be estimated using standardized response means, calculated for each SLE activity measure for the time period of most rapid change in clinical status in each patient. Changes considered important with a high degree of specificity will be identified using receiver operating characteristic curves. The most useful measure for assessing patients with SLE would be that which is both most accurate and sensitive to change. Identifying the degree of change in SLE activity that patients commonly consider important would provide a guide by which to judge the value of specific treatments.