Five centres in Great Britain and the Republic of Ireland have collaborated to produce a computerized index of clinical disease activity in systemic lupus erythematosus, based on the principle of the physician's intention to treat. The index assesses separately eight organ-based systems. The index has proved quick and easy to use despite a comprehensive database and compares favourably with two other indices of disease activity. It has great potential for use in multicentre studies of disease outcome and new therapies in systemic lupus erythematosus.