This ongoing Phase-2, randomised, placebo-controlled, double-blind study evaluated the efficacy, safety and pharmacokinetics of intravenous belimumab in childhood-onset systemic lupus erythematosus (cSLE).Patients (5 to 17 years) were randomised to belimumab 10 mg/kg intravenous or placebo every 4 weeks, plus standard SLE therapy. Primary endpoint: SLE Responder Index (SRI4) response rate (Week 52). Key major secondary endpoints: proportion of patients achieving the Paediatric Rheumatology International Trials Organisation/American College of Rheumatology (PRINTO/ACR) response using 50 and '30 alternative' definitions (Week 52), and sustained response (Weeks 44 to 52) by SRI4 and Parent Global Assessment of well-being (Parent-global). Safety and pharmacokinetics were assessed. Study not powered for statistical testing.Ninety-three patients were randomised (belimumab, n=53; placebo, n=40). At Week 52, there were numerically more SRI4 responders with belimumab versus placebo (52.8% vs 43.6%; OR 1.49 (95% CI 0.64 to 3.46)). PRINTO/ACR 30 alternative (52.8% vs 27.5%; OR 2.92 (95% CI 1.19 to 7.17)) and PRINTO/ACR 50 (60.4% vs 35.0%; OR 2.74 (95% CI 1.15 to 6.54)) responses were more frequent with belimumab than placebo, as were sustained responses for SRI4 (belimumab, 43.4%; placebo, 41.0%; OR 1.08 (95% CI 0.46 to 2.52)) and Parent-global (belimumab, 59.1%; placebo, 33.3%; OR 3.49 (95% CI 1.23 to 9.91)). Serious adverse events were reported in 17.0% of belimumab patients and 35.0% of placebo patients; one death occurred (placebo). Week-52, geometric mean (95% CI) belimumab trough concentration was 56.2 (45.2 to 69.8) µg/mL.The belimumab intravenous pharmacokinetics and benefit-risk profile in cSLE are consistent with adult belimumab studies and the 10 mg/kg every 4 weeks dose is appropriate.NCT01649765.