The autoantibodies secreted by B lymphocytes have recently been shown to play an important role in autoimmune disease. B lymphocyte depletion by rituximab, a monoclonal anti-CD20 antibody, has been introduced for the treatment of several autoimmune disorders. Few reports have underlined its potential use for the treatment of systemic lupus erythematosus (SLE). We report here the occurrence of extracapillary glomerulonephritis associated with a thrombotic event shortly after rituximab treatment for a lupus flare-up in a patient with anticardiolipin antibodies. This observation suggests that rituximab alone may be insufficient to control severe SLE with glomerulonephritis and should therefore be used with caution in patients with this condition.