This study examines the occurrence, nature and distribution of disease flares in systemic lupus erythematosus (SLE) according to organ involvement. One-hundred-and-fourteen patients were seen in the Lupus Clinic of the Bloomsbury Rheumatology Unit over a 3-yr period. At each visit a data sheet was completed to assess their disease activity in eight separate organs/systems according to the British Isles Lupus Assessment Group (BILAG) activity index. This information was entered into a computer and a score for each organ system was obtained. The record of each individual patient was examined to identify flares in the individual organs; 458 flares occurred in 107 patients. The majority of patients (69%) experienced more than one flare during the study period. Fifty-four per cent of patients had flares in more than one system simultaneously, but the majority (70%) of flares involved only one system. The most severe organ involvement (A score) was most commonly observed in the musculoskeletal system, whereas severe renal disease occurred only three times. These results indicate that although SLE is a multisystem disease, the flares that occur tend to be confined to one system at a time. Further, these results demonstrate that in our rheumatology practice the most common 'A' flare observed was severe polyarthritis (A score in musculoskeletal).