The frequency of the luteinized unruptured follicle (LUF) was determined in a population of 220 regularly cycling women, infertile for at least two years. Laparoscopy was performed during the very first days of the luteal phase. In 26 women without other demonstrable cause of infertility, a diagnosis of LUF was made based on the absence of an ovulation stigma and the low concentrations of progesterone (P) and 17 beta-oestradiol (E2) in peritoneal fluid (PF). Twenty of these 26 women underwent a culdocentesis 72-96 hours after the serum LH-rise in a following cycle.