Lymphoproliferative syndrome with well differentiated lymphocytes and moderate lymphocytosis in the peripheral blood includes a heterogeneous group of disorders, that present often difficulties in classification. We have studied the lymphocyte markers (ER, EMR, sIg and T3, T4, T8 antigens) in 36 cases who had lymphocytic infiltration in the bone marrow and peripheral lymphocyte counts less than 15 X 10(9) l-1. Four cases (11.1%) had the characteristics of T8 lymphocytosis and 31 had a B cell monoclonal proliferation in the peripheral blood. Of these, four were sIg-, EMR+, 19 were sIg+, EMR+ and 8 were sIg+, EMR-. Most patients (17/32) had the clinical picture of stage 0 and I B-CLL. Six cases presented as pure splenomegalic form of CLL, three had the features of immunocytic lymphoma and five had the features of lymphocytic lymphoma. It is concluded that the majority of lymphoproliferative disorders presenting with moderate lymphocytosis represent early forms of B-CLL. Occasionally cases of lymphocytic or immunocytic lymphoma may present problems of differential diagnosis since there may be a dissociation of phenotypic characteristics of lymphocytes between tissues and peripheral blood.