Serum CA 125 levels were studied in 2544 healthy women. Menopausal status and a history of hysterectomy were both highly significant influences on the CA 125 level (P less than .0001 for both factors). A history of hormone replacement therapy was also highly significant in reducing CA 125 levels (P = .002) in postmenopausal women. Other factors such as parity, history of unilateral oophorectomy or oral contraceptive usage, or family history of ovarian cancer were not significant influences when analysis of covariance was performed. When premenopausal women were assessed separately, age was significant (P = .04). The number of years since menopause did not influence CA 125 levels. Both hysterectomy and menopausal status have a clear effect on serum CA 125 levels and must be considered if serum CA 125 is to be used as a screening test.