A 63-year-old male patient with extensive carcinoma in situ of the urinary bladder was found to have unsuspected transitional cell carcinoma of the prostate. Mapping of the totally embedded radical cystectomy specimen demonstrated diffuse, multifocal, epithelial abnormalities, ranging from mucosal atypia to the nonpapillary carcinoma in situ with extension to the urethra, prostatic ducts and glands, seminal vesicles and ureter, probably reflecting individual urothelial susceptibility in reaction to carcinogenic stimulus. The importance of prostatic assessment in the evaluation of the patient with carcinoma in situ of the urinary bladder is emphasized.