Flow cytometry deoxyribonucleic acid measurements of bladder washings, routine bladder irrigation cytology and cystoscopic results were compared in a prospective fashion during 33 months of clinical urology practice. Of the 204 patients (286 specimens) studied 74 had bladder tumors or a history of bladder tumors and 130 had other genitourinary pathological conditions, including hematuria, cystitis, benign prostatic hypertrophy or prostatic carcinoma. Flow cytometry and cytology agreed with the cystoscopic findings in 93 per cent of the cases, whereas flow cytometry agreed with cytology in 84.6 per cent. When combined, flow cytometry and cytology were negative in only 1 case when several papillomas or grade I papillary carcinomas were present on cystoscopy, yielding a diagnostic accuracy of 99 per cent. The false negative rates for flow cytometry and cytology were 4.3 and 5.2 per cent, respectively, with false positive rates of 4.2 and 3.1 per cent, respectively. As expected the specimens that were missed by cytology were low grade tumors and those missed by flow cytometry were usually ulcerated invasive tumors. We believe that flow cytometry is a valuable adjunct to cytology and cystoscopy in the diagnosis, management and followup of patients with known or suspected bladder cancer. In addition, criteria for the diagnosis of cancer are changed such that specimens with a deoxyribonucleic acid tail and a large amount of hyperdiploid cells without a distinct stem line are considered suspicious and indicative of bladder pathology but they are not synonymous with the presence of carcinoma.