Cysts and tumors of the rete ovarii have been described only rarely and little is known of their pathology. We describe herein 16 cases of cysts (cystadenomas), 2 solid adenomas, and 1 adenocarcinoma of rete ovarii origin. The patients with the cysts were 23-80 years of age (mean age, 59 years). The cysts averaged 8.7 cm in diameter and were characterized by a location in the hilus; a flat, cuboidal, or columnar, usually nonciliated-lining epithelium; crevices along the inner surfaces; and a fibromuscular wall that often contained hyperplastic hilus cells. The cysts showed many similarities to microscopic dilatations of the rete. The rete adenomas were incidental microscopic findings in two women 45 and 68 years of age. They were located in the hilus and were composed of cords and tubules lined by cells resembling closely those of the normal rete. The rete adenocarcinoma occurred in a 52-year-old woman, was bilateral, and could be partly localized to the hilus. Microscopic examination revealed retiform spaces containing papillae and nests of transitional-like epithelium. Criteria for distinguishing cysts and tumors of rete origin from other ovarian tumors are discussed.