Transitional cell carcinoma of the bladder is a major cause of cancer deaths. Recently, much attention has been focused on ABO(H) antigen deletion in terms of prediction of prognosis. Furthermore, several studies have shown a correlation between T-antigen (a precursor of blood MN glycoprotein) expression in carcinomas of the breast, colon, and stomach. We have studied 56 specimens from 41 patients with transitional cell carcinoma of the bladder for T-antigen expression and ABO(H) antigen deletion. Results were analyzed with respect to tumor grade, tumor stage, and clinical course. The data indicate that T-antigen expression was not completely useful prognostically; it did not correlate with grade, stage, or clinical course. ABO(H) antigen expression or deletion was found to be a better predictor of tumor behavior than tumor grade, despite a false negative rate of 20 to 30 per cent in blood group O patients. We suggest that use of immunoperoxidase techniques will increase the sensitivity in group O patients, thus making ABO(H) deletion a useful predictive parameter of tumor aggressiveness. This is currently being evaluated in our patients.