To determine whether nerve fibers in the functional layer endometrium are caused by an endometriosis itself or a common symptom of pain, endometrial tissues from 30 women with endometriosis, 40 women with adenomyosis, 41 women with uterine fibroids, and 47 endometriosis women with adenomyosis were stained immunohistochemically using the highly specific polyclonal rabbit antiprotein gene product 9.5 (PGP9.5) and monoclonal mouse antineurofilament protein. We demonstrated PGP9.5-immunoactive nerve fibers in the functional layer of endometrium in women with pain symptoms, but not in women without pain symptoms, whether the women had endometriosis, adenomyosis, uterine fibroids, or endometriosis with adenomyosis, suggesting a role of PGP9.5-immunoactive nerve fibers in the functional layer of the endometrium playing in pain generation in these disorders.