Monoclonal gammopathy (IgG, lambda) in a 37-year-old man with Hashimoto's thyroiditis, was markedly decreased after thyroidectomy. Histological examination of the thyroid showed large lymphoid follicles surrounded by a massive proliferation of plasma cells. Immunological studies revealed that interfollicular plasma cells stained monotypically for lambda chain, findings in keeping with a diagnosis of plasmacytoma of the thyroid. In addition, there was kappa monotypic staining of the lymphoid follicles and absence of tangible body macrophages, indicating the presence of a follicular lymphoma derived from a separate cell line from the plasmacytoma.