Objective The aim of the present study was to describe the demographic, clinical and immunological characteristics of patients with late-onset (≥50 years) SLE vs patients with early-onset SLE (<50 years). Methods We performed a cross-sectional retrospective study of 3619 patients from the RELESSER database (National Register of Patients with Systemic Lupus Erythematosus of the Spanish Society of Rheumatology). Results A total of 565 patients (15.6%) were classified as late-onset SLE and 3054 (84.4%) as early-onset SLE. The male-to-female ratio was 5:1. Mean (s.d.) age at diagnosis in the late-onset group was 57.4 (10.4) years. At diagnosis, patients with late-onset SLE had more comorbid conditions than patients with early-onset SLE; the most frequent was cardiovascular disease (P <0.005). Furthermore, diagnostic delay was longer in patients with late-onset SLE [45.3 (3.1) vs 28.1 (1.0); P <0.001]. Almost all patients with late-onset SLE (98.7%) were Caucasian. Compared with early-onset SLE and after adjustment for time since diagnosis, patients with late-onset SLE more frequently had serositis, major depression, thrombotic events, cardiac involvement and positive lupus anticoagulant values. They were also less frequently prescribed immunosuppressive agents. Mortality was greater in late-onset SLE (14.3% vs 4.7%; P <0.001). Conclusion Late-onset SLE is insidious, with unusual clinical manifestations that can lead to diagnostic errors. Clinical course is generally indolent. Compared with early-onset disease, activity is generally reduced and immunosuppressants are less commonly used. Long-term prospective studies are necessary to determine whether the causes of death are associated with clinical course or with age-associated comorbidities in this population.