The quantitative assessment of cellular DNA and RNA content by flow cytometry to provide useful information for both diagnosis and prognosis of patients with hematologic malignancies is reviewed. While the characterization of cell surface antigens seems to be more germane to questions of the normal cell counterpart (stage) of malignant transformation and the biology of regulation of proliferation and differentiation by cell-cell contact and humoral factors, DNA-derived and RNA-derived parameters were surprisingly sensitive in the distinction of major morphologic groups, drug sensitivity and long-term prognosis. Our findings to date in the study of leukemias, lymphomas and myelomas are summarized.