Objectives of our work were to determine whether the presence of endometriosis, stage and the activity of the disease influence the sperm-peritoneal fluid (PF) incubation test and, possibly, whether a GnRH agonist (GnRHa) treatment in these patients can reduce the PF toxicity toward sperms. Preovulatory PFs from 18 patients with endometriosis (11 stage I-II rAFS and/stage III-IV; 10 with active lesions and 8 with inactive disease), and from 15 women with unexplained infertility were collected and frozen at the time of diagnostic laparoscopy. The ten patients with active endometriosis were treated by 3 months ovarian desensitization by a GnRH a (Decapeptyl 3.75 mg, Ipsen, Milan Italy) followed by a controlled ovarian hyperstimulation with a purified FSH (Metrodin, Serono, Rome, Italy) and HCG 10.000 UI (Profasi, Serono) when appropriate. In these patients, immediately before performing intrauterine and/or intraperitoneal insemination, a small aliquot of PF was collected transvaginally checking the absence of oocytes. Five hundreds suspension for one hour. PFs inducing a decrease in sperm motility greater than 20% were defined "toxic". The prevalence of toxic PF was not statistically greater in endometriosis (8/18) than in unexplained infertility (3/15) and was not dependent upon the stage of the disease, while it was significantly (rho = 0.01) more frequent in patients with active endometriosis (7/10) than other women (4/23). After a three months GnRHa treatment of these patients the rate of toxic PFs was significantly (p = 0.02) decreased (1 out of 10 treated cases).(ABSTRACT TRUNCATED AT 250 WORDS)