A 55-year-old female with T-cell large granular lymphocytic leukemia (T-LGL) (CD8+) was initially treated with anti-thymocyte globulin and then cyclosporine due to anemia/neutropenia. While the severity of cytopenia varied with the therapy, the T-LGL persisted. Eight years after the initial diagnosis, she developed lymphadenopathy and hepatosplenomegaly. A complete blood cell count revealed leukocytosis, anemia and thrombocytopenia with ∼ 80% lymphocytes. In contrast to the LGL cells, the blood lymphocytes at this time were medium-large in size and had oval/irregular nuclei, condensed chromatin, indistinct nucleoli and a moderate amount of basophilic cytoplasm, many with elongated vacuoles, and some with cytoplasmic projections. The abnormal lymphocytes comprised ∼ 30% of the bone marrow cellularity with interstitial infiltrates/aggregates. Immunophenotypic analyses demonstrated a T-cell neoplasm with features suggestive of T-cell prolymphocytic leukemia (T-PLL) (CD4+). Cytogenetic analysis revealed a novel clone with complex abnormalities. PCR-based TRG gene rearrangement studies detected a clonal amplicon distinct from that of the preexisting T-LGL. Because of the chronological sequence of the two T-cell neoplasms, this case was initially considered an aggressive transformation of T-LGL. However, this was ultimately excluded by a discordant CD4-subset restriction and the presence of a distinct clonal identity. While these two T-cell neoplasms may have intrinsic connections, the underlying pathogenesis remains to be investigated.