Background: Although the Papanicolaou (Pap) smear was originally designed to detect premalignant cervical lesions and cancer, it has become apparent that malignant cells from extrauterine primaries may also appear in the smears. Here, we report a case of a serous borderline tumor (SBT) of the ovary that was detected by a conventional Pap smear. Case Report: In a 34-year-old woman, who was operated on for a left borderline ovarian tumor 2 years ago, glandular dysplasia was discovered in a Pap smear screening. The Pap smear showed a large number of atypical glandular cells forming 3-dimensional papillary clusters. The cell clusters were enlarged and had variably sized round or oval nuclei. The background was clean, and this finding pointed to the noncervical origin of the malignant cells. A right ovarian cystic mass was discovered and a laparotomy performed. A histologic examination of the ovarian mass revealed a SBT of the ovary. Conclusion: This case illustrates the value of the Pap smear in discovering unsuspected extrauterine malignancies, including SBTs of the ovary.