Endometriosis, disorder characterized by the presence of endometrium outside the uterus cavity, is associated with chronic pelvic pain. Nerve growth factor (NGF) participates in development, repair and survival of neurons. We evaluated NGF and nerve fibres in eutopic and ectopic endometria from women with endometriosis and in normal endometrium from control patients without endometriosis. In endometrium collected during surgery for endometriosis (paired eutopic and ectopic endometria, proliferative n = 6 and secretory n = 7 phases) or during tubal ligation or hysterectomy for no endometrial disease (control, proliferative n = 6 and secretory n = 6 phases), NGF and neurofilament (NF) of nerve fibres were studied by immunohistochemistry measuring integrated optical density (IOD). Cytoplasmic NGF was detected in glands and stroma in all control, eutopic and ectopic endometrial samples; no statistical differences were found throughout the menstrual cycle. However, total (gland plus stroma) NGF IOD was higher in eutopic than in ectopic endometria during the secretory phase. NF was detected only in ectopic endometrium being eutopic and control endometria negative to this antibody, and no differences were observed between proliferative and secretory phases. Negative and significant correlation was found between NGF and NF expression in ectopic endometria only during the secretory phase. In conclusion, our results show a significant negative correlation between NGF and myelinated nerve fibres in secretory ectopic endometria. NGF expression was not significantly modified throughout the menstrual cycle in control and in endometriosis, even higher in eutopic than in ectopic tissues; the NGF secretion from lesions to pelvic cavity cannot be ruled out, and could explain the negative correla-S. Venegas et al. 418 tion observed. Although NF and NGF are not useful as diagnostic markers for endometriosis, their expression may be related to endometrial physiology and pathophysiology of the disease.