Two cases of carcinoma in a diverticulum of the bladder were experienced. The first case was of a 50-year-old male who presented in February, 1981, complaining of asymptomatic microhematuria. The excretory urogram revealed a diverticulum in the left lateral aspect of the bladder which was causing shift of the lower ureter to the median side. The cytology report of voided urine was class III. Diverticulectomy was performed and pathologic findings was a transitional cell carcinoma, grade 1, stage 0. The patient has been free of recurrence for the past 54 months. The second case was of a 67-year-old male with the chief complaint of pollakiuria. Non-papillary tumor in a diverticulum of the bladder was found by cystoscopy and computed tomography. Tumor biopsy and urinary diversion by ileal conduit were performed in the usual manner. The pathologic finding was transitional cell carcinoma of grade 11 malignancy. The patient died of intestinal obstruction on January, 19, 1984, about 15 months after the surgery. The 117 cases of carcinoma in a diverticulum of the bladder we found in the Japanese literature are reviewed briefly.