The origin of physiological CA-125 serum levels, which in normally menstruating women were shown to depend on their actual menstrual cycle phase, has not yet been completely elucidated. It is furthermore conceivable that physiological CA-125 sources may contribute to serum elevations in the various pathologies associated with increased circulating CA-125. The present review deals with menstrual cycle-dependent expression of CA-125 in normal tissues of the female reproductive tract in relation to the actual circulating CA-125 levels together with in vivo data concerning the inductive effect of medroxyprogesterone acetate on circulating CA-125 studied in 24 postmenopausal women. Furthermore, in vitro results on constitutive, steroid hormone- and cytokine-modulated CA-125 shedding from human peritoneal mesothelial and ovarian surface epithelial cells are summarized.