Investigations of human B-cell malignancies have generally focused on the monoclonal B-cell populations. Until recently there has been little emphasis on the thymus (T) lymphocyte in these disorders. Current studies, however, suggest that quantitative and qualitative disorders of T cells are generally seen both in chronic lymphocytic leukemia and in multiple myeloma. This review will focus on two major concepts. First, it will define the quantitative and functional T-cell abnormalities in B-cell malignancies including evidence suggesting a causal link between the T-cell abnormalities and certain observed disease manifestations in chronic lymphocytic leukemia and multiple myeloma. Secondly, it will review data demonstrating that these T cells may be influenced by in vivo and in vitro manipulations and will outline some of the possible resultant clinical effects.