Mature cystic teratomas (MCTs) of the ovary are typically composed of a cyst lined with neoplastic tumor cells recapitulating ectodermal differentiation including derivatives such as epidermis, dermis, and appendages (sebaceous glands), although mesodermal and endodermal derivatives can also be seen. After the observation of a meningothelial proliferation similar to ectopic meningothelial hamartoma (EMH) in a consult case of MCT and subsequently within dermal tissue in 2 consecutive MCTs, we wanted to assess the frequency of this finding and to consider its possible significance. Twenty-five consecutive cases of ovarian MCT (patient age ranging from 23 to 66 years, mean: 36.3 years; size ranging from 2.5 to 11.5 cm, mean: 6.1 cm) diagnosed in 2008 were retrieved from the archives at Brigham and Women's Hospital. All cases contained tissue that could be localized to the head and neck region including skin with abundant pilosebaceous units (scalp skin), brain, cartilage, bone, teeth, respiratory/sinonasal epithelium, and thyroid tissue. Ten of the 25 cases (40%) contained an EMA-positive meningothelial proliferation akin to EMH, including features such as psammomatous calcifications and pigmented dendritic cells. In all 10 cases, the meningothelial element was immediately adjacent to ectodermally derived skin with pilosebaceous units and glial tissue. This study shows that the presence of meningothelial tissue in MCT of the ovary is quite frequent and its appearance is similar to that of EMH. The similar morphologic appearance of the meningothelial proliferation in MCT to EMH, its localization to the dermal subcutaneous portion of MCT, and its frequent proximity to glial tissue supports the hypothesis that the tissue elements of ovarian MCT are primarily of the head and neck type (eg, scalp skin, brain, upper respiratory/sinonasal, and less commonly thyroid) suggesting that the neoplastic growth of MCT parallels normal anterior embryonic plate development with primarily the formation of the cranial (cephalad) portion of the embryo.