CA 125 is expressed by eutopic and ectopic endometrium. In women with advanced endometriosis, plasma concentrations are increased towards the end of the luteal phase and during menstruation but not during the follicular and early luteal phases. In women without endometriosis, such cyclic changes of CA 125 in plasma are not observed. In women with cystic ovarian endometriosis, plasma CA 125 concentrations are markedly elevated. Measurement of CA 125 in ovarian cyst fluid is the method of choice to differentiate a cystic corpus luteum from an ovarian endometriotic cyst, a frequent and difficult clinical problem. CA 125 can be used to diagnose deeply infiltrating endometriosis with a sensitivity of 36% and a specificity of 87%. These figures underestimate the clinical importance, since plasma CA 125 concentrations are mainly important for the diagnosis of deeply infiltrating endometriosis types II and III, which are the most severe forms and which are clinically easily missed. Because of the strong association of deep endometriosis and pelvic pain, the assay of CA 125 in plasma may be advocated in all women with unexplained pelvic pain as an aid in the diagnosis of deeply infiltrating endometriosis. Following surgical excision of endometriosis, CA 125 can be used to monitor the completeness of surgery.