We report a case of appendiceal abscess complicated with right hydronephrosis in a 67-year-old woman. She visited our department complaining of right flank pain. Intravenous pyelography (IVP) showed hydronephrosis and hydroureter on right and a narrow ureter at the pelvic brim. A pelvic computed tomographic scan revealed a low density area measuring 44 x 37 mm in size, anterior to the right ureter, which was thought to be a pelvic tumor. Further examination did not reveal the origin of the tumor. An exploratory laparotomy was performed. The tumor developed from the cecum and adhered to the right ureter. The appendix was not found. The tumor was resected with the cecum while the ureter was preserved. Histological investigations revealed an appendiceal abscess. The postoperative course was uneventful. IVP after the operation showed the hydronephrosis to have resolved.