There is not agreement about the best maintenance treatment for patients with diffuse lupus nephritis. This multicenter, randomized trial compared the safety and efficacy of cyclosporine and azathioprine. Seventy-five patients with diffuse proliferative lupus were given three intravenous methylprednisolone pulses followed by prednisone and oral cyclophosphamide for a median of 90 d. Subsequently, patients were randomly assigned either to cyclosporine or to azathioprine for 2 yr (core study). Treatment continued for up to 4 yr (follow-up study). The primary outcome measure was the incidence of disease flares. Secondary end points were proteinuria per day, creatinine clearance, and adverse effects. Seven flares occurred in the cyclosporine group, and eight occurred in the azathioprine group. At the end of the core study, mean proteinuria decreased from 2.