HCL is a lymphoproliferative disorder, primarily of B cells. T cell variant HCL is a rare clinical entity, which has a clinical picture similar to that of the common B cell HCL disease. HTLV-II has been isolated from a case of T cell variant HCL. This subtype of HTLV-II-associated disease is indolent in character in comparison to the very aggressive HTLV-I-associated disease. Investigation of the biology and molecular genetics of HTLV is in progress. It is hoped that better understanding of HTLV and the comparative differences between HTLV-I and HTLV-II will provide specific insights into the mechanisms of human leukemogenesis.