A survey of the use of flow cytometry for clinical purposes is given. In the last decade the main clinical application of this technique has been measurements of cellular DNA content for estimation of cell cycle distribution and ploidy studies. A large body of data is now available on the presence of aneuploidy in different malignant diseases. By measurements with high resolution, the demonstration of abnormal cellular DNA content in several types of t u m o r s can be of definite diagnostic value when combined with conventional diagnostic procedures. The prognostic significance of different types of DNA aberrations is so f a r not established. Attempts to monitor cancer treatment by studying altered cell cycle distribution have not been successful, although s o m e applications are of potential value. The main reasons f o r this are the complexity of t u m o r tissue as well as difficulties with interpretation of altered cell cycle distribution caused by drug combinations. For further progress in this field more emphasis on other cell constituents than DNA measured b y flow cytometry is desirable, either as single or as multiparameter measurements.