Recent reports have indicated that the neoplastic T cells of angioimmunoblastic T-cell lymphoma express CD10. It has been suggested that the demonstration of a CD10+ T-cell population may assist in establishing a diagnosis of angioimmunoblastic T-cell lymphoma and in distinguishing angioimmunoblastic T-cell lymphoma from other peripheral T-cell lymphomas. It has been unclear, however, whether this phenotypically unusual T-cell population might be present in other settings as well. In this report, we have retrospectively examined 64 cases of lymph node and solid tissue biopsies for the presence of CD10+ T cells using multicolor flow cytometry. Discrete populations of CD10+ T cells were found in 5 of 28 cases (18%) of reactive lymphoid hyperplasia, 4 of 17 cases (23%) of follicular lymphoma, and 9 of 19 cases (47%) of marginal zone B-cell lymphomas. The CD10+ T cells constituted 1-6% of total cells analyzed and </=14% of the total T-cell population. Using two-color immunohistochemical stains, many of the CD10+ PAX5-negative presumptive T cells were found to be located within germinal centers. These findings indicate that a normal small subset of CD10+ peripheral T cells exists and, at least when present in small numbers, should not be considered an indication of a T-cell neoplasm.