BLyS/BAFF (B-lymphocyte stimulator/B-cell activating factor) is a vital B-cell survival factor. Overexpression of BLyS in mice may lead to systemic-lupus-erythematosus-like (SLE-like) disease, and BLyS overexpression is common in human SLE. Treatment of SLE-prone mice with a BLyS antagonist ameliorates disease progression and enhances survival, making BLyS an attractive therapeutic target in human disease. However, several unresolved issues remain, including what is the contributory role of APRIL (a tumor-necrosis-factor superfamily member related to BLyS) in the 'autoimmunogenic' effects of BLyS, identification of the 'optimal' BLyS antagonist, and identification of those SLE patients most likely to benefit from BLyS antagonist therapy.