Frentizole is a benzimidazoleurea that has immunosuppressive properties in mice. Eleven steroid-treated patients with active systemic lupus erythematosus received frentizole (150-350 mg/day) in combination with stable or decreasing doses of prednisone in an open label trial. Nine patients completed at least one 21- to 75-day course of therapy with this drug. Clinical parameters of disease improved in 8 of these 9 patients. Mean DNA binding decreased by 28%, mean CH50 increased by 20%, and mean absolute lymphocyte and T cell counts decreased by 25-26%. Granulocytopenia was not observed. Three patients developed reversible hepatic toxicity. Clinical and serologic improvement was noted in 3 patients who accepted a second 90-day course of frentizole therapy.