Gonadotropin releasing hormone (GnRH) analogs can cause regression of hormone-dependent breast carcinomas via the specific GnRH receptor (GnRH-R). In an attempt to obtain a better understanding of GnRH actions in human breast carcinoma, the expression of GnRH-R was examined immunohistochemically in 58 invasive ductal carcinomas and correlated with various clinicopathological parameters. GnRH-R was immunolocalized in the cytoplasm of carcinoma cells in 37 of 58 invasive ductal carcinoma cases (64%). Immunoreactivity for GnRH-R was also detected focally in the cytoplasm of morphologically normal glandular epithelia adjacent to the carcinoma. A significant correlation was observed between the immunohistochemical expression of GnRH-R and estrogen receptor labeling index (LI; P = 0.030) or progesterone receptor LI (P = 0.0074). There was a significant inverse correlation between GnRH-R immunoreactivity and Ki-67 LI (P = 0.012). No significant correlations were detected between GnRH-R and other clinicopathological parameters, including patient age, menopausal status, stage, tumor size, lymph node status, histological grade and prognosis. This study indicates that GnRH-R is widely distributed in human breast carcinoma cells and regulates GnRH actions locally. Breast carcinomas positive for GnRH-R maintain some hormonal regulatory mechanisms, and GnRH actions may lead to a low proliferative rate in human breast carcinoma.