Cdc25B and cdc25A phosphatases are representative stimulators of cell cycle progression, and recent studies have also indicated their oncogenic roles. In this study, we investigated the expression of these phosphatases in malignant lymphoma of the thyroid by immunohistochemistry. These phosphatases were not expressed in follicular cells in normal follicles, but were heterogeneously or diffusely expressed in the follicles in chronic thyroiditis and malignant lymphoma. In infiltrating lymphocytes in chronic thyroiditis, they were only occasionally expressed. Of the 47 cases of lymphoma, 30 (63.8%) were classified as high group for cdc25B because it was expressed in more than 25% of lymphoma cells. Cdc25B expression level was inversely associated with MIB-1 labeling index (p=0.0008), and aberrant p53 expression (p=0.0077). Furthermore, cases of marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MZBL) were more frequently classified as high group (p=0.0318) than those of diffuse large B-cell lymphoma (DLBL). On the other hand, 22 cases (46.8%) were regarded as high group for cdc25A, but its expression level was not linked to those parameters. These findings suggest that i) cdc25B plays a role in the early phase of thyroid lymphoma possibly including the malignant transformation from chronic thyroiditis, and ii) cdc25A may contribute to the progression of lymphoma.