PIP: Focus is on some evidence of a possible relationship between the repeated involvement of the ovarian surface epithelium in the process of ovulation and the frequency of the development of the common ovarian neoplasms from this epithelium. Data about comparative ovarian oncology have been accumulating, and 3 noteworthy features have been revealed: 1) practically all tumor types encountered in the human ovary may be seen in the mammalian ovary; 2) ovarian tumors in other mammals are apparently much rarer than in human beings; and 3) this rarity is largely because of the infrequency of epithelial neoplasms derived from the ovarian surface epithelium--the cystadenoma and the adenocarcinoma. In women these account for the majority of all ovarian neoplasms and for the great majority of ovarian malignant neoplasms. Another situation has been revealed in the domestic fowl, with its frequent egg production. Adenocarcinoma of the ovary is the most common epithelial neoplasm in the entire body. The relation to egg production was demonstrated in an experiment where adenocarcinomas were induced in the ovaries of 17 out of 19 hens by maintaining them throughout life in a stable environment with 12 hours of fluorescent lighting daily. Egg production rapidly reached a maximum; it then declined over 3 years, with no seasonal rest periods. No tumors appeared in control hens kept under normal lighting conditions with seasonal variations. The surface epithelium of the ovary does not appear to play any active role in the adult processes of reproduction. The few electron microscopic studies available suggest a simple mesothelial function. The process of ovulation involves repeated minor trauma to the covering epithelium as well as repeated exposure of the ovarian surface to the estrogen rich viscous follicular fluid. Epidemiological data in human beings may be suggestive of a possible relationship between the process of ovulation and the development of the common ovarian neoplasm. In the absence of ovulation, ovarian neoplasms of surface epithelial origin are very rare, but germ cell and mesenchymal tumors occasionally arise. In patients denied the ovarian physiological rest periods afforded by pregnancies, a higher incidence of ovarian cancer has been reported. The hypothesis that the extravagant and mostly purposeless ovulations in the human female may have a contributing role in neoplasia of the surface epithelium of the ovary requires additional consideration.