The luteinized unruptured follicle syndrome (LUF) exists and is cause of infertility. Although the data of the laparoscopic inspection of the ovaries and the corpora lutea and of steroid hormone concentrations in peritoneal fluid constitute strong evidence that the syndrome exists, its role as a cause of infertility is less clear. The only evidence available that the LUF syndrome is a cause of infertility, is the finding that the syndrome occurs statistically more frequently in women with unexplained infertility than in a control group of women. It still has to be proven whether the LUF syndrome occurs repetitively in each cycle and causes infertility, or whether the syndrome occurs intermittently and only reduces the probability of conception. Diagnosis of the syndrome can be made by laparoscopic inspection of the ovaries and by the assay of 17 beta-estradiol and progesterone, in peritoneal fluid between day 14 and 20 of the cycle. The relationship between the LUF syndrome and pelvic endometriosis and luteal phase insufficiency is discussed. We suggest that the LUF syndrome might be the cause of endometriosis thus explaining the statistical association between both syndromes, and the infertility of women with only mild endometriosis. We favor the hypothesis that the LUF syndrome might be caused by stress thus constituting a "psychological infertility" and we suggest that the syndrome could explain the spontaneous cure rate. Therefore, before any therapy can be accepted as the treatment of the LUF syndrome, it should be strictly assessed with adequate controls.