We reassessed the use of DNA flow cytometry in bladder cancers on the basis of our research and already published findings. We discuss technical aspects underlying the validity of the results. Currently, the validity of DNA flow cytometry is established by parametric analysis of the DNA content of tumor cells found in the course of multiple biopsies of the tumor. In addition, we examine the results obtained with bladder washings and, in some cases, the results of biopsies of the bladder mucosa which may appear normal under cystoscopy. The complementarity of these examinations appears to be essential. Our experience confirms the results already published, suggesting that the frequency of DNA aneuploidy increases significantly according to the grade and the tumor stage. However, clinical interpretation of DNA flow cytometry results calls for some caution. There is a general consensus not to use these results in the screening of bladder cancers. However, DNA flow cytometry is particularly useful in the follow-up of carcinoma in situ since DNA aneuploidy is almost always present. DNA flow cytometry is also useful in the stratification of superficial grade 2 tumors. Finally, during the follow-up of invasive tumors, the persistence or appearance of DNA aneuploidy may be attributed to therapeutic resistance.