To investigate the expression of oxytocin (OTR) and/or vasopressin (VP1αR) receptor in patients with and without adenomyosis uteri.Retrospective nonrandomized study.University hospital endometriosis research center.Forty patients with histologically proven adenomyosis and 40 patients without adenomyosis who had undergone hysterectomy for dysmenorrhea, bleeding disorders, and fibroids.Immunohistochemical examination of both OTR and VP1αR expression in endometrium, myometrium, and adenomyotic lesions, and identification of smooth muscle cells using antibodies against OTR, VP1αR, and smooth muscle actin (sm-actin).The immunoreactive score (IRS) was used for expression of OTR, VP1αR, and sm-actin.Expression of OTR in epithelial cells of adenomyotic lesions and surrounding myometrial cells was detectable. VP1αR was expressed only in myometrial cells and blood vessels. Using a specific anti-sm-actin antibody, another spindle cell population was characterized to represent smooth muscle cells which are in direct contact with the adenomyotic stroma. Compared with the unaffected myometrium, the surrounding adenomyosis-associated myometrium overexpressed OTR and showed changes in morphology. In the uteri of patients with adenomyosis, the junctional zone was often seen to be quite fissured.In addition to the specific expression of VP1αR, OTR expression and morphologic changes in the myometrial architecture of uteri having adenomyosis support the hypothesis that dysperistalsis plays an essential role in the development of endometriosis and dysmenorrhea. In the near future, specific inhibition of this receptor might yield a promising treatment for therapy.