To evaluate the effectiveness of cyclophosphamide in the treatment of lupus nephritis, we designed a prospective study of patients with diffuse proliferative lupus nephritis. Twenty-six patients received prednisone (average dose, 40 mg per day) and 24 combined prednisone (average dose, 29 mg per day) and cyclophosphamide (average dose, 107 mg per day) for six months. Thereafter, all patients received maintenance doses of prednisone. Most of the patients improved (84 per cent) after six months of initial treatment with either program. Early progression of disease, ending mainly in end-stage renal disease, was equally frequent in the two treatment groups in patients with already advanced disease. In a four-year follow-up study there was a higher incidence (P approximately 0.04) and average rate (P approximately 0.02) of clinical recurrence of nephritis in the group initially given only steroid than in the group initially given both drugs. However, the proportion of patients alive after four years with stable or improved renal function was similar in the two treatment groups.