The cause and importance of endometriosis-associated subfertility are a subject of dispute in reproductive endocrinology. To further study this phenomenon, we have established a model to test the effect of peritoneal fluid (PF) from endometriosis patients on early reproductive events in vivo. Sexually mature female golden hamsters were subjected to an ovarian hyperstimulation protocol and divided into groups that received the following intraperitoneal injections: (1) saline, (2) human serum albumin (HSA), (3) PF from fertile controls, and (4) PF from stage I/II endometriosis patients. Animals were killed on days 4 and 17; reproductive performance was assessed by the number of oocytes and embryos recovered on day 4 and the number of uterine swellings counted on day 17. Reproductive performance was significantly impaired by PF from endometriosis patients; animals treated with control PF and HSA did not differ from control. These data demonstrate a marked impairment of early in vivo reproductive performance under the influence of endometriosis PF. These results support a role for a soluble PF component as a mediator in the pathogenesis of endometriosis-associated subfertility.