The case of a rare type of T-cell malignant lymphomas, clinically with a relatively favorable course, a T gamma-lymphoproliferative disease (T gamma-cell chronic lymphocytic leukaemia) was presented. The cytomorphological, cytochemical, immuncytochemical and cytogenetical markers and functional tests of the peripheral blood lymphocytes from the patient were tested. The leukaemic cells with light- and electron microscopy showed the so called LGL (large granular lymphocyte) morphology with multifocal reactions of acid hydrolase enzymes. These cells also expressed IgG-Fc-receptor, CD 8 monoclonal antibody positivity and a monoclonally rearranged T-cell receptor gen expression. Functionally the patient's lymphocytes developed a blastic response to the T-cell mitogen Concanavalin A (ConA), they suppressed the immunoglobulin production of B-lymphocytes in co-cultures and had a normal NK-activity but decreased ADCC values. The patient was diagnosed by blood, bone marrow and lymph node examination and does not need any therapy. This case was published because of it's diagnostic, immunological and prognostical interests.