For clinical trials of mild to moderately active systemic lupus erythematosus, a criteria for a responder is proposed. An a priori definition has a variety of advantages including the ability to compare trials with one another, expand the potential number of eligible subjects; increase sensitivity and reduce sample size requirements; exploit innovative; expedient clinical trial designs; and also quantitate a flare, as well. The availability of quantitative disease activity measures permits the development of a responder index.