B lymphocyte stimulator (BLyS) is a vital B cell survival factor. Overexpression of BLyS in mice may lead to systemic lupus erythematosus (SLE)-like disease, and treatment of bona fide SLE mice with BLyS antagonists ameliorates disease progression and enhances survival. BLyS overexpression is common in human SLE, and results from a phase I clinical trial with a BLyS antagonist in human SLE have shown the antagonist to be biologically active and safe. These features collectively point to BLyS as an attractive therapeutic target in human disease.