The course of systemic lupus erythematosus (SLE) is characterized by exacerbations (or flares) and remissions of disease activity. As part of an ongoing prospective cohort study, 3 disease activity indices, the physician's global assessment, the Lupus Activity Index, and the University of Toronto SLE Disease Activity Index, have been recorded, at least quarterly since 1987, on 185 SLE patients. We developed a definition of SLE flare and a description of its clinical epidemiology. Disease flare was defined as a change of greater than or equal to 1.0 in the physician's global assessment of disease activity (measured on a 0-3 scale) from the previous visit or from a visit within the last 93 days. Of the 185 patients, 98 (53%) had greater than or equal to 1 flare; the total number of flares was 146. The incidence of flare was 0.65 per patient-year of followup. The median time from the first study visit to a flare was 12 months. Flares were frequently characterized by constitutional symptoms, musculoskeletal involvement, cutaneous involvement, and decreasing levels of C3 and C4. At the time of flare, the mean University of Toronto SLE Disease Activity Index score increased by 3.0 and the mean Lupus Activity Index score (modified to omit the physician's global assessment) increased by 0.26. Overall, 44.8% of the flares prompted a change in treatment. Patients who experienced flares fulfilled more of the SLE criteria at entry and had been followed up for a longer duration after entry into the study, compared with those who did not have flares.(ABSTRACT TRUNCATED AT 250 WORDS)