Proliferative glomerulonephritis is a common and severe manifestation of systemic lupus erythematosus (SLE) that usually requires intensive therapy with high doses of glucocorticosteroids and cytotoxic drugs, such as intravenous (IV) cyclophosphamide (CYC). The objective of this Trial is to compare leflunomide (LEF) and azathioprine (AZA), in terms of efficacy and toxicity, as remission-maintaining treatment of proliferative lupus glomerulonephritis, after a remission-inducing therapy with a short-course IV CYC regimen. The hypothesis of this trial is that LEF is non-inferior to AZA.