Our knowledge of the relationship between minimal to mild endometriosis and subfertility is limited. The main reason for this lack of progress is most probably that the mechanism of subfertility remains to be determined. It has been claimed that mild endometriosis causes subfertility in a variety of ways--ovulation disturbances, inhibition of ovum uptake, dysfunction of the oviducts, recurrent abortions, decrease in implantation, modified response to the immune system and endoperitoneal inflammation. Currently, minimal to mild endometriosis is associated with subfertility of unknown etiology.