The aim of this study was to investigate cycle dependent changes of serum CA 125 and CA 15-3 concentrations during spontaneous ovulatory cycles. Twenty apparently healthy women with spontaneous menstrual cycles attending our infertility clinic were included. Of these women, 18 had occluded tubes as a result of sterilization. Ovulation was confirmed by luteinizing hormone test and ultrasonography and, to exclude endometriosis, a laparoscopy was performed. Serum samples for CA 125, CA 15-3, 17 beta-oestradiol and progesterone determinations were taken every second day starting on the 2nd day of the cycle until the 7th day of the next cycle. After correction for inter-individual variation in serum concentrations, highest CA 125 concentrations were found during the menstruation. During the follicular and peri-ovulatory phase CA 125 serum concentrations were lowest. For CA 15-3, serum concentrations were not statistically different throughout the cycle. CA 125 and oestradiol concentrations were negatively correlated, CA 15-3 and oestradiol concentrations were positively correlated. Absolute serum concentrations of both CA 125 and CA 15-3 vary among females. Within the female, fluctuations of CA 125 are phase related. In the population studied most of the patients had tubal obstruction and high CA 125 serum concentrations during menstruation, which revokes the theory that the menstrual rise of CA 125 is due only to retrograde menstruation.