A case of serous papillary adenocarcinoma of the ovary initially suspected to be an uretero-vaginal fistula is reported. The patient was a 57-year-old female with the complaint of urinary leakage from the vagina. Because she had undergone a supravaginal hysterectomy for myoma uteri 10 years earlier, a uretero-vaginal fistula was initially suspected. However, the fistula was not recognized from the findings of excretory and retrograde pyelographies. Chemical analysis of the vaginal discharge was similar to serum rather than urine. Gynecological examination showed the presence of an ovarian tumor. Right oophorectomy and extirpation of remaining cervix combined with partial resection of rectum was performed. From the pathological finding that the tumor consisted of serous papillary adenocarcinoma, the vaginal discharge was confirmed to originate from the tumor. Ovarian tumors with serous discharge from the vagina seem to be very rare.