Low-grade B cell lymphoma of mucosa-associated 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 and/or 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 (centro-cyte-like) cells that infiltrated tubules and showed extra-capsular 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 low-grade 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.