Current literature describes 3 different pathogenetic types of ovarian endometriotic cysts. Cortical invagination cysts arise when surface ovarian endometriotic deposits adhere to another structure (such as the broad ligament), blocking the egress of menstrual fluid produced by cycling endometriosis, which then collects and causes the ovarian cortex to invaginate. Surface inclusion cyst-related endometriotic cysts develop when endometriotic tissue colonizes preexisting inclusion cysts. Physiological cyst-related endometriotic cysts occur when endometriosis gains access to a follicle, such as at the time of ovulation. To determine whether routine histological examination is of use in the classification of endometriotic cysts, and if so, whether such classification is of clinical relevance, we reviewed the histology of endometriotic cysts of 29 women under 35 years of age. Young women were chosen so that ovarian cortex surrounding the endometriotic lining in invagination cysts could be identified by the finding of oocytes. Ten women (34%) had cortical invagination endometriotic cysts, but no inclusion or physiological cyst-related endometriomas were found. The remaining 19 women (66%) had unclassified endometriotic cysts, of which 14 (48% of total) had a fibrous wall between the endometriotic lining and medulla and 5 had extensive destruction of ovarian tissue. We concluded that cortical invagination cysts were the only common diagnosable sort of the 3 types currently being investigated and that unclassified cysts required further study to determine their pathogenesis. Our study highlights the need for a prospective study using standardized pathological and clinical methods.