The luteinized unruptured follicle syndrome is a frequent phenomenon, occurring in half of our women with regular cycles and infertility. Progesterone concentrations and 17 beta-oestradiol concentrations were assayed in peritoneal fluid of women during the luteal phase. Up to day 20 of the cycle, the concentrations were significantly higher in women with an ovulation stigma than in women without an ovulation stigma on their corpus luteum. The range of concentrations was sufficiently different in the early luteal phase to be used diagnostically, the only limitation being the presence of a cystic corpus luteum. We suggest that the assay of progesterone and 17 beta-oestradiol in peritoneal fluid should be done in all women with infertility and biphasic basal body temperature charts in order to diagnose the luteinized unruptured follicle syndrome.