OBJECTIVE Type I membranoproliferative glomerulonephritis (MPGN) is an immune-complex disease with a relatively poor prognosis. It has no established treatment in adults. Our hypothesis was that this disease would respond to B cell depletion with rituximab, an anti-CD20 monoclonal antibody. METHODS We conducted an openlabel trial, in Canada and the United States, of rituximab in 6 adult patients with Type I MPGN (4 idiopathic, 2 with cryoglobulinemia). The rituximab dose was 1,000 mg intravenously on Day 1 and on Day 15. The patients were followed for 1 year. The primary outcome was the change in proteinuria. RESULTS Peripheral blood B cells were suppressed, after rituximab, in all patients. The mean urinary protein excretion was 3.9 ± 2.0 g/d before treatment. Proteinuria fell in all patients, at all-time points, after rituximab administration. The difference was statistically significant (p < 0.05) at 6, 9 and 12 months, but not at 3 months. The minimum mean urinary protein excretion was 1.4 ± 1.4 g/d at 9 months. There were 2 complete and 3 partial remissions among the 6 patients. The creatinine clearance did not change significantly over the course of the study. There were no adverse effects. CONCLUSIONS Rituximab reduced proteinuria among patients with Type I MPGN. This trial suggests that B cells may play a role in this disease and that additional study of B-cell suppression is warranted.