Abstract： Objective To investigate the association between distribution of protein gene product (PGP) 9.5-immunoactive nerve fibers in peritoneal endometriotic lesions and disease-associated pain symptoms.Methods Thirty two peritoneal endometriotic lesions from patients with endometriosis (16 cases with pain and 16 cases without pain) and matched with 20 peritoneal tissues from patients with uterine leiomyoma without endometriosis were stained immunohistochemically for PGPg.5-immunoactive nerve fibers.Results The positive rate and density of PGP9.5-immunoreactive nerve fibers in peritoneal endometriotic leision were 62% (10/16) and (3.8 ± 1.7)/mm~2 in endometriosis patients with pain, which were significantly higher than 19% (3/16) and (1.7 ± 0.5)/mm~2 in endometriosis patients without pain (P <0.05) and 25% (5/20) and (1.3±0.6) /mm~2 in peritoneal tissues in women without endometriosis (P <0.05).However, no differences were found between endometriosis patients without pain and women without endometriosis (P > 0.05) .Moreover, the density of PGP9.5-immunoreactive nerve fibers in peritoneal lesions in endometriosis patients with pain was positively correlated with the severity of pain (r = 0.855, P < 0.05).In addition, the density of PGP9.5-immunoreactive nerve fibers in peritoneal lesions was statistically higher in endometriosis patients with chronic pelvic pain and(or) dysmenorrhea than those in endometriosis patients with other type of pain(P < 0.05), which was not associated with active lesion, site and staging (P > 0.05).Conclusion It suggested that PGP9.5-immunoreactive nerve fibers might confer the mechanism of pelvic pain with endometriosis.