This paper has discussed the evidence for the presence of infertility in patients with endometriosis and more critically reviewed some of the studies that have addressed the impact of various potential local peritoneal mechanisms that may lead to subfertility. Substantial evidence supports the notion that patients with endometriosis have reduced fecundability. Although several mechanisms, including, e.g., anatomic factors and ovulatory dysfunction, are possible, recent studies have pointed towards local inflammatory cells and their secretory products as being important mediators of subfertility. Ample evidence exists for the presence of an altered peritoneal inflammatory environment in patients with endometriosis. In addition, in vitro studies have identified peritoneal macrophages and their secretory products, specifically TNF-alpha as the most likely contributors to the reduced fecundability through effects on sperm function.