T-cell regulatory lymphocytes (Treg) are identified by their reactivity with CD4, CD25, and FOXP3, and are variably present in the background of various neoplasms including hematopoietic tumors, and function modulating the immune response, including control of autoimmunity. Adult T-cell leukemia/lymphoma is an aggressive lymphoma associated with human T-lymphotrophic virus 1 infection characterized by the presence of neoplastic lymphocytes with a Treg phenotype; however, this phenotype is not characteristically found in other lymphomas. Here, we report 2 apparently immunocompetent human T-lymphotrophic virus 1-negative patients with nodal and extranodal peripheral T-cell lymphoma, not otherwise specified with a Treg immunophenotype, based on the strong CD25 and FOXP3 positivity of the neoplastic cells. One patient was a 48-year-old woman with an early stage tumor in the cavum, who despite of chemotherapy subsequently developed systemic disease and died of tumor progression 46 months from diagnosis. The second patient was a 65-year-old male with generalized adenopathy and B symptoms who received chemotherapy achieving a complete remission but had recurrence and died 36 months from diagnosis. The histopathology revealed a diffuse infiltrate with an interfollicular distribution in the second case, with nodal involvement, consisted of large cells with clear cytoplasm associated with vascular proliferation and abundant mitoses. Neoplastic cells of first case showed typical Treg phenotype, whereas the second case had a CD4/CD8 double negative Treg variant. Only a single similar case was found in a review of the literature. We conclude that peripheral T-cell lymphoma, not otherwise specified with a Treg phenotype may represent a distinct category of T-cell lymphoma with an aggressive clinical course and poor prognosis. Copyright © 2011 by Lippincott Williams & Wilkins.