Transitional cell carcinoma of the urinary bladder is a multifocal disease and the whole bladder mucosa is more or less involved in the neoplastic process. A mapping study of 5 cystectomy specimens was performed, and cell proliferation was evaluated immunohistochemically using anti-proliferating cell nuclear antigen (PCNA) monoclonal antibody. A correlation between the pathological findings and the rates of PCNA-positive cells was observed in both the tumor and the surrounding mucosa in 4 of the 5 cases. The rates of PCNA-positive cells increased in areas with normal mucosa adjacent to those with dysplasia or transitional cell carcinoma, and in areas with normal mucosa or dysplasia adjacent to those with carcinoma in situ or transitional cell carcinoma in 2 cases. These features may indicate that a field change observed in the mapping study of a bladder with cancer is a manifestation of a stepwise progression of the mucosal disease.