We report a case of leukemic malignant T-cell lymphoma with mixed small and large cells. The small cells displayed a mature CD8-positive phenotype, a diploid DNA distribution by cell cycle analysis, and structural karyotypic abnormalities. Large cells were near triploid, showed additional structural cytogenetic abnormalities and a more immature membrane phenotype without CD8 expression. Altogether, these data provide suggestive evidence for a clonal evolution from a mature small cell T-cell lymphoma to a more immature large cell proliferation.