OBJECTIVE To elucidate the effect of oxidized low-density lipoprotein (LDL) and peritoneal fluid of women with endometriosis on monocyte chemotactic protein-1 (MCP-1) production by peritoneal mesothelial cells and endometrial cells. METHODS In vitro study. METHODS University medical center. METHODS Five women undergoing surgery for pelvic pain, infertility, or endometriosis; five women without endometriosis who were undergoing tubal ligation were the controls. METHODS Mesothelial cells and endometrial cells in culture were treated with oxidized LDL and peritoneal fluid from control and endometriosis patients, then MCP-1 levels were measured. METHODS ELISA was used to measure MCP-1 in the culture supernatants exposed to oxidized LDL and peritoneal fluid from control and endometriosis patients. Cellular MCP-1 messenger RNA expression was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) assay. RESULTS Treatment with oxidized LDL caused an increase in accumulation of immunoreactive MCP-1 in the medium of cultured mesothelial and endometrial cells (primary endometrial stromal cells and endometrial cell line EM42). The mesothelial cells secreted more MCP-1 than did endometrial cells under the culture condition. The EM42 cells cultured in the presence of peritoneal fluid from endometriosis patients secreted more MCP-1 than those cultured with peritoneal fluid from normal women. However, no differences were found in MCP-1 levels in the supernatant of endometrial stromal cells cultured with peritoneal fluid. CONCLUSIONS This is the first report of MCP-1 expression in mesothelial cells induced by oxidized LDL, and provides direct evidence of inflammatory action of peritoneal fluid of women with endometriosis.