Seven patients with thrombocytopenia and systemic lupus erythematosus were treated with intravenous (IV) doses of human immunoglobulin to assess clinical response and to examine the mechanism of action of IV immunoglobulin in these patients. Five of 7 patients had a greater than 50% increase in their platelet counts. Four of these patients had a sustained benefit of at least 6 months duration. The initial effectiveness of IV immunoglobulin therapy was not dependent on the reduction of levels of circulating platelet-binding IgG or circulating immune complexes.