The first problem concerns the diagnosis, namely that endometriosis can be asymptomatic and therefore its diagnosis requires performing an operative intervention (laparoscopy or laparotomy). Additionally, the diagnosis of endometriosis is not always histology based, and conversely, in a number of cases, normally appearing peritoneum reveals endometriotic lesions. Limiting the case definition of endometriosis to women with a laparoscopic diagnosis may introduce a selection bias. Furthermore, women with endometriosis whose symptoms are improved by first-line medical treatments may never need an invasive, albeit confirmatory, diagnosis. It has been suggested that the diagnosis of endometriosis should require the presence of symptoms, but in the diagnostic work-up of an infertile couple there is much controversy as to whether the presence of minimal endometriosis in the absence of anatomical distortion, means that the infertility should be considered as being caused by the endometriosis or as an infertility of unknown cause The populations studied also differ widely: ranging from infertile women to women with pelvic pain, women undergoing tubal sterilization, women undergoing laparoscopies because of different reasons and women unexposed to spermatozoa. It is therefore no wonder that reports concerning the epidemiology of endometriosis show a wide variation in prevalence rates and report numerous discrepancies.