Serum CA 125 levels during the menstrual cycle. Serum concentrations of CA 125 were measured in different phases of the menstrual cycle in 16 women with ovulatory and 12 women with anovulatory cycles. CA 125 levels were significantly elevated during menstruation in both groups. In women with anovulatory cycles, but not in those with ovulatory cycles, CA 125 levels were already increased in the premenstrual phase. A negative correlation was found between serum CA 125 and progesterone concentrations in the premenstrual phase of the cycle. We suggest that premenstrual elevation of serum CA 125 in women with anovulatory cycles is related to premature endometrial vascular changes which are the result of the low serum progesterone concentration leading to insufficient endometrial control. Thus the effect of progesterone seems to be indirect rather than a direct effect on CA 125 synthesis. When the CA 125 assay is used for diagnosis of cancer, sampling should not be done immediately before or during menstruation because the physiological elevation of the CA 125 levels may give false positive results. 