Rituximab, a chimeric monoclonal antibody specific for human CD20, has recently been used for the treatment of autoimmune diseases. A 14-year-old patient with severe systemic lupus erythematosus (SLE) and class IV glomerulonephritis presented with immunologic and clinical resistance to conventional immunosuppressive therapy for 10 months after diagnosis. To induce remission of active SLE, treatment with 6 monthly rituximab at 375 mg/m(2), oral mycophenolate and prednisone was initiated followed by maintenance rituximab every 3 months. The SLEDAI decreased significantly from 31 at diagnosis to 14 after nine applications of rituximab. Extrarenal symptoms of SLE improved significantly. However, after induction therapy with rituximab the patient presented a reversible intrinsic acute renal insufficiency for a period of 3 weeks. The discontinuation of the daily medication (oral prednisone and mycophenolate) by the patient herself may explain the progression of active SLE associated with the reversible acute renal failure. Under intensive immunosuppressive therapy improvement of active disease manifestations and stabilization of plasma creatinine concentrations to normal values was observed. However, proteinuria remained elevated and improved only after a protracted period (median protein-to-creatinine ratio 5.2 g/g, range 0.8-11.2 g/g). Hematuria and urinary cell casts persisted. In conclusion, the extrarenal symptoms of the patient responded particularly well to rituximab. However, despite complete B-cell elimination, renal remission of SLE was not achieved. Thus, it may be possible that humoral and cellular immune mechanisms have a fundamental involvement in the pathogenesis of SLE nephritis.