Recently, several investigators have demonstrated that the MN blood group precursor antigens Thomsen-Friedenreich antigen (T-Ag) and Tn-antigen (Tn-Ag) are expressed on the cell surfaces of several cancers, including urinary bladder cancer. T-Ag is composed of a specific carbohydrate chain, galactose-beta-1-3-N-acetylgalactosamine (GalNAc), which is specifically detectable through the immunohistochemical binding of peanut agglutinin (PNA). In normal cells, T-Ag is cryptic (cT-Ag) and can be unmasked by treatment with neuraminidase. Tn-Ag is composed of another carbohydrate chain, alpha-GalNAc-serine/threonine, and binds specifically to Vicia villosa agglutinin (VVA). With the use of both these lectins, VVA and PNA, the presence or absence of Tn-Ag and T-Ag was examined in 24 specimens of normal bladder epithelium and specimens from 53 cases of human urinary bladder transitional cell carcinoma of various histologic grades by staining paraffin sections by means of the avidin-biotin-immunoperoxidase technique. The correlation between the expression of these antigens and the patient's clinical course was then estimated. Out of 21 patients in whom the tumors expressed the phenotype Tn-Ag(+), T-Ag(+), or cT-Ag(-), 17 suffered from invasive recurrence. Although patients with tumors expressing the phenotypes T-Ag(-) and cT-Ag(+) have been reported to show a good clinical course, in our studies some of them showed a switch to invasive recurrence. Thus it was not possible to estimate the patient's clinical course only by the presence or absence of T-Ag and cT-Ag. The expression of Tn-Ag was then examined with the use of VVA. Of 38 cases expressing the phenotypes T-Ag(-) and cT-Ag(+), 6 had tumors that carried Tn-Ag; 5 of them suffered from invasive recurrence. These results indicated that the detection of Tn-Ag with the use of VVA in combination with the examination of T-Ag and cT-Ag is useful for estimating the degree of malignancy of bladder cancer and the patient's clinical course.