The radiographic appearance of ovarian dermoid cysts is generally familiar. In the most typical cases, the distinctive finding is a cluster of several calcific tooth-like structures. Recent experience with some atypical cases prompted the author to undertake a study of the material at the Lying-in and New York Hospitals. The findings of this investigation are presented. Pathology Knowledge of the structure of dermoid cysts is an important prerequisite to the understanding of their radiographic appearance. The tumors are cystic, sometimes loculated, and vary from a few millimeters in diameter to the size of a melon. They are distinguishable from teratomas by their cystic nature, but the distinction cannot always be made on a histologic basis alone. The cyst wall is usually 1 to 3 mm. in thickness. The contents consist of a thick, greasy, lardaceous material, formed by the sebaceous glands of the cyst. In rare instances, when sebaceous glands are absent—the so-called “asebaceous dermoid cyst"—a thin, serous fluid is present. Hairs and rudimentary teeth may sometimes be observed lying free within the cyst. More often, these structures arise from and are embedded in the cyst wall, frequently concentrated about a raised, nipple-like protuberance, the mammilla or dermal plug. The teeth are usually of the incisor or molar variety and may sometimes be contained in a rudimentary fragment of maxillary bone. In addition to these ectodermal structures, such mesodermal elements as cartilage and bone may occur as plaques in the cyst wall. Microscopically, most of the sebaceous and sudiferous glands and hair follicles appear in the region of the mammilla, while the remainder of the cyst wall is composed of fibrous tissue lined by squamous epithelium. Physical Considerations In the energy range of diagnostic roentgenography, the differences in atomic number of air, water, and fat result in varying degrees of x-ray absorption by those elements. Thus, the soft tissues (muscles and organs), which have an average atomic number of water, cast a grayish-white shadow on the roentgenogram. Calcific structures, with a higher average atomic number and thus greater x-ray absorption, cast a dense white shadow and are termed “radiopaque.” Air, having a low atomic number, absorbs the least amount of radiation and therefore appears black; it is “radiolucent.” Fat is of lower average atomic number than bone or soft tissue, approaching air in this respect. It therefore appears radiolucent on the roentgenogram. In view of the fatty contents of dermoid cysts, this physical characteristic of fat bears an important relationship to their radiographic demonstration. Author's Material During the period 1947 to 1953, 153 operations for ovarian dermoid cyst were done at the Lying-in Hospital (New York) and 28 at the New York Hospital.