This pooled analysis will assess data from the Phase 3 belimumab registration studies BLISS-52 (aka BEL110752) and BLISS-76 (aka BEL110751). The analysis was pre-planned and agreed prior to the unblinding of either study. The primary objective is to evaluate the impact of belimumab treatment on a more severe subpopulation of systemic lupus erythematosus (SLE) subjects from BLISS-52 and BLISS-76 to aid physicians and payers in decision making. Subjects are from the modified Intent-to-Treat (ITT) population defined as randomized subjects who received at least 1 dose of study agent. This more severe subpopulation will have renal, neurological, haematological, or cardiovascular/respiratory organ domain involvement (as defined by a British Isles Lupus Assessment Group (BILAG) domain score of A, B or C in at least one of the domains) at baseline AND anti-double-stranded deoxyribonucleic acid (anti-dsDNA) positive (≥ 30 IU/mL) at baseline OR low C3 and/or C4 complement relative to the normal range at baseline.