Blood group A, B and H antigens were investigated in 183 paraffin embedded biopsies from 58 patients with transitional cell carcinoma of the urinary bladder, by a modified specific red cell adherence test, direct immunofluorescence with Ulex Europeus lectin and indirect immunoperoxidase method with monoclonal antibodies against blood group antigens. The results were correlated with pathological grade and stage and with the clinical course of patients evaluating the recurrence index and clinical state. Histological findings were roughly correlated with the expression of red cell tissue antigens but not with the presence of precursor H substance in biopsies from patients of blood group A or B, in which a higher proportion of H positive results was appreciated. The clinical course was also related to the presence or absence of blood group antigens in referential biopsies: 90 per cent of negative biopsies corresponded to patients who had high recurrence index whereas 75 per cent of positive biopsies corresponded to patients who had low recurrence index or did not have recurrence for five years; 25 per cent of recurrences observed in patients with referential positive biopsy were invasive whereas the proportion of invasive tumor in recurrence from negative biopsies rises to 73 per cent. In addition, all the final biopsies from patients who died of bladder tumor were negative for blood group antigens. The diagnostic and prognostic significance of these tissue antigens in transitional cell carcinoma of the urinary bladder is discussed, and we conclude that the analysis of blood group antigens in bladder biopsies by established techniques is a useful tool in clinical pathology for the screening and followup of bladder tumors, as previously suggested.