The relationship of ABH(O) blood group antigen expression in intraepithelial dysplastic lesions to clinicopathologic properties of associated transitional cell carcinoma of the bladder. Paraffin-embedded, giant-step sections of 13 bladders with transitional cell carcinomas were stained with monoclonal anti-A or anti-B antibodies to investigate whether intraepithelial dysplastic lesions are related to obvious tumors. Normal and/or hyperplastic lesions were retained in only eight bladders; severe dysplasia and/or carcinoma in situ were found in all bladders except two. AB-antigen expression was retained in intraepithelial lesions of bladders with invasive carcinoma. Most intraepithelial lesions were AB-antigen negative in bladders with frequently recurrent tumors. In bladders with initially multiple tumors, AB-antigen expression was negative in almost one half of the intraepithelial lesions. Therefore, it appears likely that most multiple or recurrent bladder carcinomas arise from dysplastic cells in intraepithelial lesions which have acquired malignant potential; initially invasive tumors quickly develop from a limited lesion acquiring a high malignant potential without changes of cell phenotype in most intraepithelial lesions. 