© 2015 S. Karger AG, Basel. Copyright: All rights reserved. Background: Idiopathic membranous nephropathy remains the leading cause of nephrotic syndrome in Caucasian adults. Immunosuppressive therapy with cyclosporine (CSA) is often successful in reducing proteinuria, but its use is associated with a high relapse rate. Rituximab, a monoclonal antibody that specifically targets CD20 on the surface of B-cells, is effective in achieving a complete remission of proteinuria in patients with idiopathic membranous nephropathy. However, whether rituximab is as effective as CSA in inducing and maintaining complete or partial remission of proteinuria in these patients is unknown. The membranous nephropathy trial of rituximab (MENTOR) hypothesizes that B-cell targeting with rituximab is non-inferior to CSA in inducing long-term remission of proteinuria. Methods and Design: Patients with idiopathic membranous nephropathy, proteinuria ≥5 g/24 h, and a minimum of 3 months of Angiotensin-II blockade will be randomized into a 12-month treatment period with IV rituximab, 1,000 mg (2 infusions, 14 days apart; repeated at 6 months if a substantial reduction in proteinuria (equal to or >25%) is seen at 6 months) or oral CSA 3.5-5 mg/kg/day for 6 months (continued for another 6 months if a substantial reduction in proteinuria (equal to or >25%) is seen at 6 months). The efficacy of treatment will be assessed by the remission status (based on changes in proteinuria) at 24 months from randomization. Patient safety will be assessed via collection of adverse event data and evaluation of pre- and posttreatment laboratory data. At the 6-month post-randomization visit, patients who have been randomized to either CSA or rituximab but who do not have a reduction in proteinuria ≥25% (confirmed on repeat measurements within 2 weeks) will be considered treatment failures and exit the study. Discussion: This study will test for the first time whether treatment with rituximab is non-inferior to CSA in inducing long-term remission (complete or partial) of proteinuria in patients with idiopathic membranous nephropathy.