The lymphocytes that accompany thymomas express an immature T-cell phenotype, as usually demonstrated by CD1 or TdT immunoreactivity. Even when thymomas metastasize or occur in ectopic sites, the infiltrating T lymphocytes show this unique immature phenotype, contrasting with thymic and nonthymic carcinomas, in which the infiltrating T lymphocytes typically show a mature phenotype (CD1 and TdT negative). Therefore, the presence of an immature T-cell population in an epithelial tumor strongly supports a diagnosis of thymoma. The availability of an antibody that consistently marks immature T-cells in routine paraffin sections would be of great help in the study of thymic tumors. In this report, we describe the use of MIC2 antibody (013), which has been widely used for the diagnosis of Ewing's sarcomas and peripheral primitive neuroectodermal tumors because it intensely stains thymocytes. Immunohistochemical staining was performed on paraffin sections of normal/hyperplastic thymus (18 cases), thymoma (62 cases), thymic carcinoma (nine cases), tumors showing borderline features between thymoma and thymic carcinoma (three cases), and ectopic hamartomatous thymoma (two cases). T-cell and B-cell antibodies were also applied to aid in the interpretation. In the normal thymus, almost all lymphocytes in the cortex stained with 013, whereas fewer than 5% of those in the medulla were 013 positive. In thymomas, including the three ectopic thymomas and the single case of metastatic thymoma, most lymphocytes were 013 positive, except the spindle-cell foci (medullary thymoma or medullary component of mixed thymoma), in which the percentage of 013-positive lymphocytes was lower (5-30%). Within the pale foci of "medullary differentiation" and the perivascular spaces of lymphocyte-rich thymomas, few lymphocytes showed 013 positivity, indicating that the T lymphocytes in these areas were more mature. None of the thymic carcinomas harbored 013-positive lymphocytes. Among the three cases of borderline thymoma/thymic carcinoma, only one harbored 013-positive lymphocytes. The 013-positive lymphocytes were not seen in the ectopic hamartomatous thymomas. In normal lymph nodes and nonthymic carcinomas studied as controls, there were no or at most small numbers of isolated 013-positive lymphocytes. We conclude that interpreted in the proper context, MIC2 antibody can serve as a useful marker of immature T-cells and thus help in the confirmation of a diagnosis of thymoma in small biopsy specimens, ectopic thymoma, or metastatic thymoma; in the distinction between invasive thymoma and thymic carcinoma; and in the classification of thymomas.