Purpose: To assess the efficacy and safety of belimumab in patients with active SLE.Methods: 865 seropositive (ANA Ն1:80 and/or anti-dsDNA Ն30 IU/ mL) SLE patients with SELENA SLEDAI (SS) Ն 6 (on stable standard of care [SOC] therapy Ն30 days) were randomized and treated in this phase 3, 52-wk double-blind, placebo (plc)-controlled international trial of belimumab (1 or 10 mg/kg) plus SOC or plc plus SOC.Patients were dosed on Days 0, 14, 28, then every 28 days for 48 wks.Adverse events (AEs) were assessed at every visit.Efficacy analyses included the SS, BILAG and SS Flare Index (SFI).The primary endpoint was the Wk 52 SLE Responder Index (SRI): improvement in SS (Ն 4 point decrease), no new BILAG A or 2 B flares, and no Ͼ0.3 point worsening in Physician's Global Assessment (PGA) vs. baseline.Results: Mean baseline characteristics across treatment groups were similar: age 35.5 yrs, female 94.9%; race: Caucasian 26.5%, Asian 37.8%, Indigenous American 32.3%; SLE disease duration 5.3 yrs; SS 9.8; BILAG 1A/2B 58.0%; ANAϩ 94.5%; anti-dsDNAϩ 74.5%; low C4 59.3%, proteinuria (Ͼ0.5 g/24hr) 16.8%, antimalarials 67.2%, prednisone Ն7.5 mg/day 69.4%,immunosuppressants 42.2%.SRI response rates were 51.4% (pϭ0.013) in the 1 mg/kg and 57.6% (pϭ0.0006) in the 10 mg/kg belimumab dose groups (vs.plc 43.6%).Significant improvement was seen in at least one of the belimumab treatment groups for: 1) SS Ն4-point reduction, 2) improvement or no Ͼ0.3 point worsening in PGA, 3) reduction in prednisone use, and 4) reduction in flare rates (all and severe) by SFI and new BILAG 1A or 2B and increase in time-to-first flare (Table ).The rates of deaths, serious AEs and infections, and lab abnormalities were comparable between belimumab treatment groups and plc (Table ).Serious or severe infusion reactions were modestly increased with belimumab compared with plc.No malignancies were reported.