Flow cytometry is a classic laser technology. With the discovery of the cytometer, flow cytometry has become a primary tool in biodiagnostic research. This review focuses on current applications of flow cytometry to the diagnosis of disease and treatment monitoring at the single-cell level. A description of the principles of flow cytometry and a brief overview of the major applications are presented. Our criteria for selecting research papers for this review are those that show advances in biomedicine and pharmacotherapy achieved by using non-invasive flow cytometry. New concepts for diagnosis and classification based on quantitative measurements of cellular parameters and the expression of specific differentiation antigens on the surface of cells will be discussed herein.