Endometriosis, one of the most prevalent gynaecological disorders, may affect fertility. Extensive research has been done in an attempt to understand the pathogenesis of endometriosis and its association with reproductive failure. It has been suggested that the disease affects almost any step of reproduction, but data are mostly controversial so it is difficult to draw clear conclusions from studies that have been done so far. Not only is peritoneal fluid in close proximity to endometriotic lesions, but it is also the environment in which early reproductive events take place. Studies on the peritoneal fluid in endometriosis have provided significant data towards an understanding of this disease. Immunological factors play a key role in determining the occurrence of endometriosis as well as its heterogenous symptoms. Since data also indicate that there are immunological differences between infertile and fertile women with endometriosis, recent studies have been designed to take these differences into consideration. This review will discuss the mechanisms by which endometriosis may affect fertility, and an emphasis will be placed on the relevance of the peritoneal fluid.