A review of the literature concerning the analysis of nuclear DNA content by flow cytometry is made and it is compared with own experience. The advantages and the limits of this technique are examined. The practical problems in the interpretation of the histograms and the value of the measurements are discussed. The importance of this analysis in diagnosis and in staging of many tumors and the clinical involvements are emphasized. In many tumors DNA ploidy represents a new independent prognostic variable that is useful to separate the cases with a potential worse behaviour in an early stage, when other classic parameter are not available. It is also stressed the importance of flow cytometric DNA analysis of tumor cells. This is made on the same tissues that pathologists use for histopathological diagnosis and the results are important in many cases for a diagnostic accuracy.