We recently observed a unique case of T-cell type chronic lymphocytic leukemia, documented by cell surface marker analyses. Immunologic tests at the time of diagnosis were as follows: 1) skin tests were negative; 2) in vitro lymphocyte responses to antigens or pokeweed mitogen were absent; 3) in vitro lymphocyte responses to phytohemagglutinin, concanavalin-A or allogeneic cells were low but significant; and 4) stimulating capacity of leukemic T cells were absent. Chromosomal analysis of bone marrow showed a pattern of 45 chromosomes with a marker chromosome. Terminal deoxynucleotidyl transferase activity was very low. Patient responded extremely well to COP (cyclophosphamide-oncovin-prednisone) therapy. Patient died of recurrent transitional cell carcinoma while his leukemia was in complete remission, approximately 20 months after the diagnosis of T-cell CLL.