OBJECTIVE: To devise a more discriminating version of the British Isles Lupus Assessment Group (BILAG) disease activity index and to show that it is reliable. METHODS: A nominal consensus approach was undertaken by members of BILAG to update and improve the BILAG lupus disease activity index. The index has been revised following intense consultations over a 1-yr period. It has been assessed in two real-patient exercises. These involved patients with diverse clinical features of SLE, including gastrointestinal, hepatic and ophthalmic problems, which the earlier versions of the index did not fully take into account. Reliability in terms of the ability to differentiate patients was assessed by calculating intraclass correlation coefficients. The level of agreement between physicians was determined by calculating the ratio of estimates of the standard error (SE) attributable to the physicians to the SE attributable to the patients. RESULTS: Good reliability and high levels of physician agreement were observed in one or both exercises in the constitutional, mucocutaneous, neurological, cardiorespiratory, renal, ophthalmic and haematological systems. In contrast, the musculoskeletal system did not score as well, although providing more clear-cut glossary definitions should greatly improve the situation. CONCLUSIONS: Some significant changes in the BILAG disease activity index to assess patients with SLE are proposed. The process of demonstrating validity and reliability has started with these two exercises assessing real patients. Further validation studies are under way. BILAG 2004 is likely to be valuable in clinical trials assessing new therapies for the treatment of SLE, as it provides a more comprehensive system-based disease activity measure than has been available previously.