In 100 consecutive patients who were undergoing laparoscopy for infertility (group 1, n = 52), chronic pelvic pain (group 2, n = 18) or tubal sterilization (group 3, n = 30, asymptomatic fertile women), peritoneal biopsies were taken from areas of visually normal peritoneum of uterosacral ligaments. Twenty-six patients in group 1 (50%), eight patients in group 2 (44.4%) and 13 patients in group 3 (43.3%), were found to have laparoscopic evidence of endometriosis elsewhere in the pelvis. The majority of women (80.7% in group 1, 87.5% in group 2, and 100% in group 3) had stage I disease. The incidence of the distinctive appearances of the lesions was similar in the three groups of patients and 7% of all women or 15% (7/47) of those patients having endometriosis at laparoscopy had only subtle (non-¿typical') endometriotic peritoneal lesions. Uterosacral biopsies showed the presence of endometriotic tissue in three cases (5.7%), two cases (11%) and three cases (10%) in groups 1, 2, and 3 respectively. One of the two patients in group 2 and two of the three patients in group 3 had no evidence of endometriosis at laparoscopy; thus histological study revealed the presence of endometriosis in normal peritoneum in 11% (5/47) of patients having macroscopic endometriosis and in 6% (3/53) of patients without endometriosis at laparoscopy. Previous oral contraceptive users were significantly higher among women having macroscopic and/or microscopic endometriosis than among women without the condition. In conclusion, our prospective study shows a high prevalence (45-50%) of endometriosis (including microscopic forms) in both patients with chronic pelvic pain and asymptomatic women (fertile and infertile), thus supporting the modern concept that in many women endometriosis may be a paraphysiological condition while probably only in some patients small amounts of endometriosis are an ¿annoyance' with implications to their reproductive health and may produce symptoms (e.g. pelvic pain) and therefore should be defined as a ¿dis-ease'. Previous use of oral contraceptives may increase the risk of developing endometriosis.