What is the source of elevated serum levels of CA 125 in patients with endometriosis?

The presence of CA 125 was investigated in endometriotic tissue and endometrial mucosa using two different immunohistochemical methods, indirect immunofluorescence and the avidin-biotin immunoperoxidase technique. Neither of the methods revealed the antigen in the 12 endometriotic fragments studied. The endometrial mucosa both of patients with endometriosis and of the control subjects was positive with indirect immunoperoxidase staining in all cases, whereas with indirect immunofluorescence it was positive in 75 and 67%, respectively. Thus the mechanism(s) responsible for distribution of the antigen in different tissues and its reported high serum levels in advanced endometriosis is still unclear.