The case histories of 263 patients with histologically proven endometriotic cysts of the ovaries (1962-1979) were studied. The diagnosis frequency increased continuously during the observation period. Most patients were aged 35-39 years. The admission diagnosis was rarely endometriosis. The clinical picture and the extent of the disease rarely showed correlation. At laparotomy 12% of the chocolate cysts had ruptured spontaneously (31 of 263) and half of the cysts ruptured during surgery without adverse effects on the postoperative course. The recurrence rate in patients treated by conservative surgery was 7%. The most frequent accompanying diseases were uterine fibroids (42%) and adenomyosis (42%). One third of the patients presented with bilateral endometriotic cysts. Further foci of endometriosis tissue were frequently found in the affected ovary (55%) and in the other pelvic organs (43%). Only 4% of the ipsilateral oviducts were closed at their fimbrial end, and only 10% showed signs of endosalpingitis. The incidence of infection in endometriotic cysts--formation of an isolated ovarian abscess--was between 8% and 18%, while the risk of carcinoma was less than 1%.