To study the immediate effects of high-dose intravenous methylprednisolone pulse therapy on the immune mechanisms in the absence of other immunosuppressive agents, we treated ten patients with active systemic lupus erythematosus, six with renal and three with central nervous system involvement with three daily 1 gram intravenous doses of methylprednisolone and measured the immune response before and just after discontinuation of the drugs. After the treatment, the mean seum IgG, IgA and IgM levels were essentially unchanged. Likewise, serum C3 and C4 levels were not changed significantly. In nine of ten patients, methylprednisolone pulse therapy reduced the levels of circulating immune complexes (p < 0.05). Thus the immediate clinical improvements with methylprednisolone pulse therapy are suggested to be the result of depression of the circulating immune complex levels.