Several clinical indices have been proposed to measure disease activity in systemic lupus erythematosus (SLE), a disorder characterised by alternate phases of flare and remission. Over the last 5 years a European multicenter study was carried out in order to reach a consensus on the definition of SLE activity. A new index, ECLAM (European Consensus Lupus Activity Measurement), was created and then validated in the first and in the second parts of the study, respectively. In addition, a comparison between ECLAM and the other most commonly used lupus activity indices was performed. ECLAM appeared to be the best activity index for classifying lupus patients, whether used as a single state index (i.e., to measure activity at a given moment in time) or as a transition index. (i.e., to measure variations in activity over time).