OBJECTIVES: We aimed to describe juvenile-onset systemic lupus erythematosus (jSLE) features and to establish its differences compared to adult-onset SLE (aSLE) from a large national database. METHODS: Data from patients (≥4 ACR criteria) included in Spanish Society of Rheumatology Lupus Registry (RELESSER) were analysed. Sociodemographic, clinical, serological, activity, treatment, cumulative damage, comorbidities and severity data were collected. Patients with disease onset <18 years were described and compared to those with disease onset ≥18 years. RESULTS: We reviewed 3,428 aSLE patients (89.6% women) and 484 jSLE patients (89.8% girls), 93% Caucasian (both groups). Mean age at diagnosis was 38.1±14 and 16.6±6.3 years (p<0.001) and mean age at the end of follow-up was 48.8±14.3 and 31.5±30 years (p<0.001), respectively. jSLE showed significantly more clinical (including lymphadenopathy, fever, malar rash, mucosal ulcers, pericarditis, pleuritis, Raynaud's phenomenon, lupus nephritis, recurrent nephritis, histologic nephritis changes, thrombocytopenia, haemolytic anaemia, thrombotic thrombocytopenic purpura, seizures, lupus headache and organic brain syndrome) and immunological (a-dsDNA and a-Sm antibodies, hypocomplementaemia) involvement than did aSLE, except for secondary Sjögren's syndrome, a-Ro antibodies, fibromyalgia and osteoporosis. jSLE also showed more SLE family history, longer diagnosis delay, higher SLEDAI and Katz scores, but lower Charlson scores than aSLE. Several specific domains were more frequently involved in SLICC/ACR DI in jSLE. jSLE patients more frequently underwent all SLE-related treatment and procedures, as well as dialysis and kidney transplantations. CONCLUSIONS: jSLE shares many clinical and serological features with aSLE. However, jSLE patients typically manifested more activity, severity, cumulative damage in certain areas, than their aSLE counterparts.