The main aim of this study is to describe the in vivo temporal and spatial expression of monocyte chemotactic protein 1 (MCP-1) in human endometrial endothelial cells (HEECs) and to compare the in vitro regulation of MCP-1 expression by sex steroids in HEECs from women with or without endometriosis. Eutopic endometrial tissues and endometriosis implants were grouped according to the menstrual cycle phase and were examined by immunohistochemistry for MCP-1 expression. No significant difference was observed for MCP-1 immunoreactivity in the endothelial cells of eutopic endometrium of women with endometriosis when compared to endometrium of women without endometriosis and to endometriosis implants. For in vitro studies, the purity of cultured HEECs (90%-95%) was confirmed by immunocytochemistry using endothelium-specific markers CD31 and CD146. The effects of estradiol (5 x 10(- 8) mol/L), progesterone (10(-7) mol/L), or both on MCP-1 messenger RNA (mRNA) and protein levels were analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis and enzyme-linked immunosorbent serologic assay (ELISA), respectively. Sex steroids did not have significant effect on MCP-1 mRNA and protein expression in HEECs from women without endometriosis. However, we observed that the sex steroid treatment stimulated MCP-1 mRNA and protein expression in HEECs from women with endometriosis (P < .05). We postulate that the stimulation of chemokine expression by sex steroids in the endometrial endothelial cells in women with endometriosis may play a central role in recruiting mononuclear cells, therefore contributing to the inflammatory aspect of endometriosis.