OBJECTIVES: The aim was to characterize disease severity, clinical manifestations, treatment patterns and flares in a longitudinal cohort of adults with SLE in the UK. METHODS: Adults with SLE were identified in the Clinical Practice Research Datalink-Hospital Episode Statistics database (1 January 2005-31 December 2017). Patients were required to have ≥12 months of data before and after the index date (earliest SLE diagnosis date available). SLE disease severity and flares were classified using adapted claims-based algorithms, which are based on SLE-related conditions, medications and health-service use. RESULTS: Of 802 patients, 369 had mild, 345 moderate and 88 severe SLE at baseline. A total of 692 initiated treatment in the first year after diagnosis. Five hundred and fifty-seven received antimalarials, 203 immunosuppressants and 416 oral CSs. Information on biologic use in hospitals was unavailable. The mean (S.d.) time to initiating any medication was 177 (385.3) days. The median time to first flare was 63 days (95% CI: 57, 71). At least one flare was experienced by 750 of 802 patients during follow-up; the first flare was mild for 549 of 750, moderate for 116 of 750 and severe for 85 of 750. The mean (S.d.) annual overall flare rate (year 1) was 3.5 (2.5). A shorter median time to first flare was significantly associated with moderate/severe disease (P < 0.001) and clinical manifestations (P < 0.001). CONCLUSION: Our findings suggest some delay in the initiation of SLE treatment. Most patients experience a flare within 2 months of diagnosis. Early treatment might delay or reduce the severity of the first SLE flare and might translate to slower disease progression, lower accrual of organ damage and better outcomes.