We report a case of adenocarcinoma of the appendix invading the urinary bladder in a 75-year-old man. Although cystoscopic examination and computed tomography suggested a primary or secondary bladder tumor, repeated transurethral bladder biopsy could not confirm the neoplasm. At operation a primary neoplasm of the appendix invading the bladder was discovered and en bloc resection of the urinary bladder with the adherent cecum followed by an ileocolonic anastomosis and ureterocutaneostomy was performed. The patient died of carcinoma 13 months later.