A European Consensus Group study, involving 29 centres from 14 countries, was performed in order to reach agreement on the definition of disease activity in systemic lupus erythematosus (SLE) and to construct a new disease index. Data on 704 lupus patients were collected and analysed, using univariate and multivariate statistical procedures, to select those clinical and laboratory features of the disorder which best correlate with the global assessment of disease activity assigned to the patients by the physician of each participating centre. A combination of 15 clinical and laboratory variables was shown to be the best predictor of disease activity in SLE. A European Consensus Lupus Activity Measurement (ECLAM) was then formulated. This index included the 15 selected variables, weighted (with some adjustments) according to their respective regression coefficients in the multivariate model. ECLAM appears to be an effective instrument for scoring patients with different degrees of disease activity. This is the first SLE disease activity index based on data from a very large number of lupus patients followed at a large number of lupus centres in different countries. It might therefore very well serve as a standardised measure for future European clinical studies. Final assessment of the validity, reliability and sensitivity of this index is now underway.