This Journal feature begins with a case vignette that includes a therapeutic recommendation. A discussion of the clinical problem and the mechanism of benefit of this form of therapy follows. Major clinical studies, the clinical use of this therapy, and potential adverse effects are reviewed. Relevant formal guidelines, if they exist, are presented. The article ends with the author's clinical recommendations. A 20-year-old woman was evaluated by her rheumatologist because she was disabled by flares of systemic lupus erythematosus (SLE). A diagnosis of SLE had been made 2 years earlier, on the basis of a photosensitive malar rash, oral ulcers, polyarthritis, pericarditis, and positive assays for antinuclear antibodies (ANA) and anti-double-stranded DNA (dsDNA) antibodies. Treatment with analgesics and hydroxychloroquine for 6 months was not beneficial; prednisone at a dose of 40 mg daily resulted in some improvement, but the patient gained 6.8 kg (15 lb) and required two hospitalizations for infections. SLE flares occurred when the prednisone dose was less than 30 mg daily. Trials of methotrexate, mycophenolate mofetil, and azathioprine either had unacceptable side effects or failed to control flares or permit prednisone tapering. On examination, she had cushingoid features with 20 swollen, tender joints and 3 oral ulcers. The ANA titer was positive, at 1:320. An assay for anti-dsDNA antibodies was negative, and the serum complement level was normal. The rheumatologist recommended a trial of belimumab.