The expression of variant isoforms of the adhesion molecule CD44 is correlated with the onset of neoplasia in many carcinomas. We have previously shown that noninvasive detection of bladder carcinoma is possible by analysis of anomalous CD44 expression in exfoliated urothelia. Although the sensitivity and specificity values obtained for the detection of bladder tumors using RT-PCR and Western blotting methods were superior to those obtained using urine cytology, the application of such techniques is inconvenient for routine diagnostic use. We now report the design and development of a sandwich-ELISA system for the reliable detection of CD44 protein extracted from sedimented urothelial cells in voided urine. Naturally micturated urine samples were obtained from 53 patients with newly diagnosed bladder cancer and from 65 subjects with no evidence of disease; patients with gross hematuria were excluded because of interference with the assay. To demonstrate the diagnostic potential of the system, a "gate" was imposed at N (max), i.e., the highest absorbance value obtained from a sample known to be tumor free. All values above this value were assumed to be indicative of the presence of a tumor. Using this parameter, 42 of 53 (81.1%) patients with histologically confirmed bladder tumors were correctly diagnosed. Correspondingly, under these conditions, the assay is 100% specific for tumor detection, with a sensitivity of 81.1%, which equates to a positive predictive value of 100% and a negative predictive value of 81.1%. A further 54 patients who had previously received treatment for bladder cancer but were currently clinically disease-free were also investigated. Of these, 47 of 54 (87%) were correctly diagnosed to be tumor-free, which in this group equates to a positive predictive value of 87% and a negative predictive value of 100%. The data presented demonstrate that the rapid and accurate detection of elevated levels of CD44 protein isoforms in exfoliated urothelial cells is applicable to the identification and monitoring of primary and recurrent bladder cancer.