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We report fine-needle aspiration cytology (FNAC) obtained from two cases of mucosa-associated lymphoid tissue (MALT) lymphoma with extreme plasma cell differentiation. The patients were 61-year-old and 69-year-old Japanese women presenting with thyroid swelling. The smears contained numerous plasma cells, lymphocytes with plasma cell differentiation and scattered centrocyte-like (CCL) cells. In addition, one case demonstrated occasional atypical giant plasma cells. Occasional intranuclear inclusions (Dutcher bodies) of the plasma cells were observed in the other case. A few cytologic lymphoepithelial lesions-clusters (originating from lymphoepithelial lesions in histology) were also observed in one case. Plasma cells occupied ˜10% in all of the lymphoid populations in FNAC specimens of Hashimoto thyroiditis, whereas, both cases demonstrated ˜45% plasma cells and lymphocytes with plasma cell differentiation of all lymphocytes.The cytomorphologic findings of both cases were similar to those of plasmacytoma of the thyroid. However, immunohistochemical and flow cytometry studies demonstrated that both cases were MALT lymphoma with extreme plasma cell differentiation. From a therapeutic perspective, it is important to discriminate MALT lymphoma from plasmacytoma.