OBJECTIVE To evaluate the impact of progesterone in the peritoneal fluid on human sperm motility in the infertile patients with endometriosis. METHODS Peritoneal fluids were collected from 41 patients by laparoscopy, of which 17 were infertile patients with endometriosis, 11 were fertile patients with endometriosis and 13 were fertile controls. The concentrations of progesterone in the peritoneal fluids were measured by the immulyze-enzyme amplified luminescence system. After 4 h of incubation in vitro with the collected peritoneal fluids, sperm motility was assessed by the computer-assisted analyzer (CASA). RESULTS The concentrations of progesterone in the peritoneal fluids in the patients (infertile patients with endometriosis, fertile patients with endometriosis and the fertile controls) did not differ significantly (P > 0.05) in the proliferate phase. The sperms incubated with the peritoneal fluids from the patients with endometriosis had significant lower sperm motion parameters (A, VCL, VSL, VAP, ALH) than those of the fertile controls (P < 0.05). Although the sperm motility incubated with the peritoneal fluids from the patients with endometriosis was enhanced by the exogenous progesterone, no significant correlation was established between the concentrations of progesterone in the peritoneal fluids and the sperm motility (P > 0.05). CONCLUSIONS The peritoneal fluids of infertile patients with endometriosis inhibit the sperm motility. The inhibition is not associated with the concentrations of progesterone in the peritoneal fluids. However, the sperm motility can be enhanced by adding exogenous progesterone to the peritoneal fluids.