membranoproliferative glomerulonephritis associated with lowgrade b cell lymphoma presenting in the kidney lowgrade b cell lymphoma of mucosaassociated tissue type maltoma rarely may involve the kidney membranoproliferative glomerulonephritis mpgn is an uncommon complication of b cell lymphoma and may be related to cryoglobulin andor immunoglobulin synthesis by a secretory b cell clone we report 2 patients with the novel renal biopsy findings of coexistent maltoma and mpgn both subjects presented with nephrotic proteinuria and renal insufficiency one patient had a serum m protein igg k but neither individual had any other clinical or serologic evidence of systemic disease including hematolymphoid malignancy autoimmune disease cryoglobulinemia or hepatitis c viral infection both renal biopsies demonstrated mpgn type i with immunoglobulin deposits that in 1 case showed light chain restriction igm k electron microscopy disclosed corresponding glomerular electron dense deposits in subendothelial locations both biopsies also contained atypical interstitial lymphoid infiltrates comprising marginal zone centrocytelike cells that infiltrated tubules and showed extracapsular extension immunostains demonstrated a predominantly b cell population that lacked expression of cd5 and cycline d1 and gene rearrangement studies confirmed the presence of a monoclonal b cell population in both cases these findings indicate that lowgrade b cell lymphoma in the kidney may be an unexpected finding in patients with nephrotic syndrome related to mpgn immunophenotypic and gene rearrangement studies are important ancillary tools for the evaluation of atypical lymphoid infiltrates in kidney biopsies