To determine CA-125 concentrations and total amounts in peritoneal fluid (PF) of women with various infertility-related factors throughout the menstrual cycle. Peritoneal fluid was obtained at laparoscopy. CA-125 was determined using the assessed two-step immunoradiometric assay (IRMA) which, in contrast to the one-step IRMA, gives valid results. University Hospital Nijmegen, Nijmegen, The Netherlands. One hundred six infertile women with a regular and ovulatory cycle were included. None. The mean PF CA-125 concentration and total amount were significantly lower during the luteal phase as compared with other phases of the menstrual cycle. No correlation was found with the presence or absence of endometriosis, adhesions, a male and/or cervical mucus infertility factor, and with patent or closed fallopian tubes. Peritoneal fluid CA-125 concentrations varied from 630 to 12,000 arbitrary units/mL (mean +/- SD = 3,437 +/- 2,286). Total PF CA-125 amounts (concentration x PF volume) varied from 1,760 to 13,300 arbitrary units (mean +/- SD = 30,219 +/- 26,841). CA-125 secretion into the abdominal cavity varies during the menstrual cycle. Retrograde menstruation is not the main source of CA-125 in PF.