To evaluate the clinical effectiveness of intermittent intravenous cyclophosphamide in the treatment of severe systemic lupus erythematosus, 20 patients with systemic lupus erythematosus (SLE) and evidence of severe renal involvement or systemic vasculitis, consecutively admitted to the hospital were studied. Cyclophosphamide was administered intravenously at a dosage of 1.0 g/m² monthly, during 6 months and maintained every 3 months during 12 additional months. Of 10 patients with active lupus nephritis, a reduction or disappearance of proteinuria and maintenance of normal renal function was recorded in 6. Improvement of renal function was observed in 4 out of 7 patients with renal insufficiency at initial evaluation; resolution of renal insufficiency was more frequently observed in patients with recent onset renal failure. At the end of the follow-up (18.0 ± 14.5 months) disappearance or reduction of nephrotic range proteinuria was recorded in 6 out of 14 patients; there was progression toward renal failure in 4 patients (20%). Response to intravenous cyclophosphamide therapy was observed in 4 of 5 patients with severe extrarenal SLE. Side effects, recorded in 12 patients, were mild and transient and in no patient was the treatment discontinued. Four patients died during the follow-up, although in 2 of them the deaths were not attributable to therapy. Even though this was an open and uncontrolled study, intermittent, intravenous cyclophosphamide was an effective therapy for severe, steroid refractory SLE.