The application of flow cytometry to cancer diagnosis and the prediction of tumor behavior continues to expand. The ability of flow cytometry to examine multiple parameters of large numbers of individual cells is being exploited increasingly to characterize neoplastic processes better. This will allow closer examination of tumor heterogeneity and identification of subpopulations with different behavioral patterns. Flow cytometry is being used with greater frequency in attempts to predict tumor behavior and response to therapy. Flow cytometry may have the most to offer in this area. Diagnosis of cancer by routine histopathological examination will not be replaced by flow cytometry; flow cytometry will be used in conjunction with morphological descriptions. The detection of proliferation antigens and oncogene products, as well as cell cycling, provides information on neoplastic progression that is not readily obtainable by other methods. In addition, flow cytometry will undoubtedly be used to measure other features of neoplastic cells, such as enzyme levels and ion fluxes, which may better characterize the behavior of the tumor. Advances in flow cytometry instrumentation, light sources, fluorochrome development, and basic aspects of cellular and molecular biology will continue to permit this technology to define neoplastic cells and their behavior better, resulting in both improved patient care and a better understanding of tumor biology.