Background: Although nerve fibres are present in eutopic and ectopic endometrium, it is unclear whether they appear in ovarian endometriotic lesions. We investigated the presence of nerve fibres in ovarian endometriotic lesions and its correlation with clinical parameters in women with ovarian endometriosis. Methods: Histological sections of ovarian endometriotic lesions from 61 women with ovarian endometriosis (Stages II-IV) who underwent laparoscopic endometrioma cystectomy were stained immunohistochemically using a specific polyclonal rabbit anti-protein gene product 9.5 (PGP9.5) antibody to demonstrate myelinated and unmyelinated nerve fibres. Results: Nerve fibres stained with PGP9.5 were detected in ovarian endometriotic lesions in 31.1 of women, and most appeared in fibrotic interstitium of ovarian endometriotic lesions. The density of PGP9.5-immunoactive fibres in ovarian endometriotic lesions in women with pain symptoms (n = 35) was higher than in women with no pain symptoms (n = 26, P = 0.039), although the percentage (positive cases/total) of PGP9.5-positive fibres did not differ. In women with pain symptoms, PGP9.5-positive fibres appeared in 40.0 of cases and the density of PGP9.5-immunoactive fibres in ovarian endometriotic lesions was correlated with severity of pain symptoms (r = 0.466, P = 0.005). In women with no pain, PGP9.5-positive fibres were detected in only 5 (19.2) women. Both the percentage and the density of PGP9.5-positive fibres in ovarian endometriotic lesions were associated with pelvic adhesions (χ² = 6.833, P = 0.009; Z = 2.442, P = 0.015, respectively) but not with disease severity. Conclusions: PGP9.5-immunoactive nerve fibres in ovarian endometriotic lesions may be involved in the pathophysiology of pain generation and pelvic adhesion formation in women with ovarian endometriosis.