Monocytoid B-cell lymphoma arising in extranodal organs. Six cases of monocytoid B-cell lymphoma (MBCL) developing in extranodal sites (thyroid, three; stomach, two; rectum, one) are described. Patients were all women aged 46 to 65 years (median, 53 years). Three patients with thyroid lymphoma presented with an increasing goiter, two with positive serum antithyroid antibodies, and one patient had chronic lymphocytic thyroiditis (CLTH). The histologic type of surgically resected specimens was a diffuse proliferation of atypical lymphoid cells with monocytoid appearance, i.e., abundant pale cytoplasm with distinct cell border and small reniform nucleus. These cells were CD20+, 22+, 24-, 9- showing their B-cell origin. The monoclonal nature of the proliferating cells was confirmed by restricted expression of immunoglobulin (Ig) light chain and/or gene rearrangement study in three cases. Two cases of thyroid lymphoma in which the monoclonality could not be confirmed had histologic appearances characteristic of malignant lymphoma. All three patients with gastric or rectal lymphoma had reactive lymphoid hyperplasia (RLH) near the tumors. These findings showed presence of MBCL in the extranodal sites with invariable coexistence with lymphoid follicles formed by CLTH in thyroid or RLH in stomach and rectum. 