Background: Mature cystic ovarian teratoma (MCOT) is the most common primaryovarian tumor. Rarely, MCOT may undergo malignant transformation. Melanoma arising primarily in MCOTis a rare event. Melanocytic nevi have also rarely been reported in MCOT. Case Report: A 28 year-oldfemale presented with a palpable, 4.6 cm, right pelvic mass on physical examination. Histologically,the cystic neoplasm demonstrated epidermis with numerous pilosebaceous units and respiratory-type epithelium(endoderm) surrounded by adipose tissue and cartilage (mesoderm). A 2.0x1.0 cm pigmented area correspondedto a nevus with architectural and cytologic features of the so-called "dysplastic nevus," including variable-sizednests of nevomelanocytes irregularly distributed on distorted rete ridges, bridging of nests betweenrete ridges, fibroplasia around rete ridges, and junctional shouldering beyond the dermal nevus. Thenevomelanocytes demonstrated moderate cytologic atypia. Diagnostic criteria of melanoma were not identified.Conclusions: Herein, we report, to the best of our knowledge, the first case of an atypical ("dysplastic")nevus, arising in a MCOT. No features of melanoma were present and the patient is disease-free at one-yearfollow-up. Rarely, melanocytic nevi and melanomas arise from the ectodermal component of MCOTs. Moreover,melanomas may arise de novo or in association with a nevus. Distinction between a melanocytic nevus,as in our case, and a primary melanoma is critical for determining the patient's prognosis and need foradditional therapy. As primary ovarian melanomas, like their skin counterpart, may arise from a precursorlesion, removal of a melanocytic nevus, such as this atypical nevus, could theoretically prevent melanomatransformation.

Keywords: Neoplasms, Multiple Primary - pathology, Nevus, Pigmented - pathology, Ovarian Neoplasms - pathology, Teratoma - pathology