Ten patients with chronic lymphoid leukemia were analyzed for clinical characteristics, morphology and phenotype of leukemic cells. There were 3 patients with T-chronic lymphocytic leukemia (CLL), 2 with T-prolymphocytic leukemia (PLL), 2 with B-CLL, 1 with B-PLL, 1 with non-T-non-B-CLL and 1 with Waldenström's macroglobulinemia. Although chronic lymphoid leukemia is usually characterized by proliferation of B-lymphocytes, our study revealed that 5 of 10 patients had T-cell phenotype. A peripheral blood specimen of T-CLL showed small lymphocytes with a mature appearance and an irregular nuclear margin. Most of the cells lacked large azurophilic granules in the cytoplasm and nucleoli were also inconspicuous in the nucleus by light and electron microscopic examinations. In PLL, a majority of the cells were large lymphocytes with a prominent nucleolus and abundant basophilic cytoplasm which were more clearly observed by transmitted electron microscopic examination. The patients were treated with cyclophosphamide and prednisolone, but response to treatment was transient. The median survival time was 7 months for all patients, while that of T-cell lineage cases was only 1 month. Therefore, new modalities of treatment must be investigated in the future.