Thirteen cases of idiopathic chronic lymphocytosis are the subject of this report. Patients showed a lymphocyte count between 4 and 15 X 10(9)/L for at least six months, marrow lymphocytosis not exceeding 25%, absence of lymphomegaly and hepatosplenomegaly, and no associated infections, immune, or neoplastic disease. Morphologic examination of smears revealed a lymphocytosis of large granular lymphocytes in five. A selected battery of polyclonal and monoclonal antisera to antigens commonly found on B-, T-, and natural killer cells allowed the identification of six cases of early B-cell chronic lymphocytic leukemia, two cases of T-cell lymphocytosis with a suppressor or helper T-cell phenotype, and five cases of large granular lymphocyte/natural killer cell proliferative disease. The results demonstrate the usefulness of combining morphologic and phenotype studies for the investigation of chronic lymphocytosis, which often appears as an early leukemia or a benign clonal proliferative disorder of lymphocyte subsets.