The flow cytometric studies presented herein are based almost entirely on DNA measurements and represent an early application of this diagnostic test. Nevertheless, the MSKCC experience with FCM has demonstrated that it is technically feasible and clinically useful. The sensitivity of FCM is in the range of 80% to 85% overall, and is superior to that of conventional voided or bladder wash cytology. In the absence of inflammation secondary to infection, calculi, or intravesical agents such as BCG, the specificity is greater than 90%. In the presence of inflammation, FCM appears to be less specific than conventional cytology. A potential advantage of FCM over cytology is the quantitative nature of the examination permitting comparisons of sequential examinations. Also, there are refinements in technique that hold promise of increasing the accuracy or clinical usefulness of FCM, eg, the joint measurements of DNA and differentiation antigens defined by monoclonal antibodies. Yet, despite the current and potential advantages of FCM, voided urinary cytology continues to be the procedure of choice for detection and monitoring urothelial carcinoma, not only because of our long experience and better understanding of this test, but because of its proven high specificity, because it is noninvasive, widely available, and may help to detect upper tract or urethral tumors.