Over the last few years cytological investigations of bladder lavage have gained ever-increasing importance in the diagnosis and follow-up of bladder tumors. Flow cytometric DNA analysis is searching for more objective ways to characterize tumor tissue beyond its morphological differentiation. Since the end of 1987 more than 400 bladder lavages have been analyzed both cytologically and by multiparameter flow cytometry. DNA/cytokeratin-8,18 antibody labelling of methanol-fixed single cells provides a standardized method which is not liable to disturbances and enables use in a routine laboratory. Aneuploidy was divided in several subgroups according to the DNA index. Proliferation of the urothelial population as a diagnostic parameter was investigated. Multiparameter flow cytometric measurement can identify and assess all subpopulations of bladder lavage such as inflammatory cells, squamous cells and squamous cell metaplasia. Cytokeratins enable a selective examination of the urothelial population after gating; the diagnostic accuracy for aneuploid tumor stem lines is markedly increased when compared to single-parameter DNA analysis. Higher specificity is reached by excluding falsely positive ‘aneuploid’ squamous cells; higher sensitivity is made possible by lowering the limit of detection for actual aneuploidy within a specimen, especially for near-diploid and tetraploid carcinomas. When compared to cytological malignancy grading, the aneuploidy rate is 26% in G1/2 tumors, 42% in G2 and 78% in G3 tumors (p < 0.005). More than half of the aneuploid bladder lavages with a negative or suspect cytology were diagnosed as tumor recurrences either histologically or cytologically in the following year. Moderately differentiated carcinomas with an aneuploid DNA distribution had a higher rate of recurrence than tumors with euploid distributions when treated curatively by organ-conserving therapy. DNA/cytokeratin analysis of bladder lavage enables an artefact-free measurement of tumor criterium aneuploidy; this is more specific and less sensitive than cytology. Due to the lack of separability of urothelial proliferation in euploid specimens, the aim to make flow cytometry a method equaling cytology cannot be reached. Therefore, the greatest value of flow cytometry is not in tumor screening, but in reliable detection of highly malignant aneuploid tumors at initial diagnosis, during therapy and recognition of recurrence of superficial bladder carcinomas. Reviewing the literature, flow cytometrically proven aneuploidy, especially triploid tumor stemlines, can be used to predict possible invasive growth. DNA/cytokeratin measurements are hence indicated when an exact assessment of prognosis can influence the therapeutic procedu