A patient with a high level serum monoclonal IgM lambda paraprotein (3,850 mg/dL) was found to have a mass infiltrating the gastric mucosa. Gastric biopsy with immunohistochemical stains showed a B-cell lymphoplasmacytoid infiltrate expressing IgM lambda, consistent with Waldenström's macroglobulinemia. The patient's response to gastric radiation indicated that the primary source of the macroglobulinemia was the stomach. This is an extremely rare presentation, with only six other reported cases of this entity with gastric involvement.