Mature T-cell leukemias are a group of neoplasms derived from mature or post-thymic T-cells, and a number of distinctive disease entities have been defined in the World Health Organization (WHO) classification. Here we report a 54-year-old female patient with multi-lobated atypical cells expressing the classic T-cell antigens involving multiple lymph nodes, peripheral blood, and bone marrow. The clinical, laboratory, and pathologic features of her disease did not fit into any of the entities in the WHO Classification. There was no evidence of rapidly rising lymphocyte counts, TCL1 expression, eosinophilia, erythroderma, Sezary cells, autoimmune phenomena, cytotoxic granules, nor evidence of HTLV-1 infection, and thus, T-cell prolymphocytic leukemia, Sezary syndrome, T-cell granular lymphocytic leukemia, and adult T-cell leukemia/lymphoma were all ruled out. This case suggests that further characterization and definition of the "unclassifiable" cases of mature T-cell neoplasm is needed to better understand the group of disorders.