33 infertile women with normal ovulatory cycles were investigated for the presence of a Luteinized Unruptured Follicle Syndrome (L.U.F.) using steroid hormone assays in peritoneal fluid and laparoscopic visualization of ovulation stigmata. We failed to identify a stigma in 36% (12) of the patients in the early luteal phase, 1 subject had a cystic corpus luteum and in 4 cases no diagnosis was made due to the presence of adhesions. The mean hormone concentrations in PF were significantly higher when the stigma was present (17-beta-estradiol, P less than 0,05; progesterone, P less than 0,01; 17-oh-progesterone, P less than 0,05). The two groups (with and without ovulation stigmata) showed no differences in plasma levels of Estradiol (E2) and Progesterone (P). Stigmata were detected only in 17% of subjects with concomitant endometriosis. 3 patients with a luteal phase defect showed low levels of steroids in PF in spite of the presence of an ovulation stigma.