Mucosa-associated lymphoid tissue (MALT) lymphomas usually arise at sites of acquired MALT and are uncommon in native MALT (eg, Peyer patches and tonsil). Malignancy in these low-grade lymphomas is often inferred by immunoglobulin light-chain restriction and expression of CD43; molecular genetic evidence is sought only if these are in doubt. We report 6 cases (4 tonsils, 2 appendixes) of marginal zone (MZ) hyperplasia in children aged 3 to 11 years that, despite histologic and immunophenotypic features indicative of lymphoma, were polyclonal by molecular analysis. No lymphoma-directed therapy was given and patients remain alive and well (5 cases, median follow-up 35.3 months). The involved tonsil and appendix showed florid MZ hyperplasia with prominent intraepithelial B cells (IEBCs). The MZ B cells and IEBCs showed a high-proliferation fraction and a CD20⁺, CD21⁺, CD27^-, immunoglobulin (Ig) superfamily receptor translocation-associated 1-positive (IRTA-1⁺), CD43⁺, multiple myeloma oncogene 1 (MUM-1), IgM⁺D⁺ phenotype. Polymerase chain reaction (PCR), cloning, and sequencing of rearranged IgH and Igλ genes (whole tissue sections [6 cases]; microdissected cells [2 cases]) showed that the MZ B cells and IEBCs were polyclonal and the IgH genes nonmutated. In contrast, MZ (intraepithelial) B cells of 6 control tonsils had a similar immunophenotype, except for expression of CD27 and polytypic light chains, whereas molecular studies showed that they were polyclonal with mutated Ig genes. (Blood. 2004;104:3343-3348)