Laparoscopy is currently considered to be the gold standard investigation in patients suspected to have endometriosis, but this is an invasive and relatively costly procedure and there may be significant delays in diagnosis. As the eutopic endometrium is recognized to be abnormal in patients with endometriosis, it has been suggested that endometrial sampling could provide an indirect diagnostic approach. In particular, recent reports have suggested that the presence of nerve fibers within the endometrial functional layer could represent a specific and sensitive marker of concurrent peritoneal endometriosis. However, such studies have been performed in select patient groups and using novel sampling and analytic techniques that are not used routinely in clinical pathology laboratories. The present study was performed upon conventional endometrial biopsies from 68 patients who underwent laparoscopy for suspected endometriosis. The biopsies were stained immunohistochemically for the neural marker PGP 9.5 and examined in a blinded manner. Endometrial functional layer nerve fibers were identified in 15 (22%) biopsies overall including 9/47 (19%) cases with histologically confirmed peritoneal endometriosis and 6/21 (29% cases) without endometriosis. There was no correlation between the presence of functional layer nerve fibers and the presenting symptoms, endometrial histology, or current hormonal therapy. In our experience, endometrial functional layer nerve fibers assessment performed using standard immunohistochemical techniques on routine biopsy specimens proved neither sensitive nor specific for the diagnosis of endometriosis. Pathologists and gynecologists considering this diagnostic approach should carefully consider the methodological factors that may influence its reliability. © 2013 International Society of Gynecological Pathologists.