Objective To determine whether high levels of interleukin (IL)-10 can attenuate the production of tumor necrosis factor (TNF)-α–induced proinflammatory cytokines in endometriotic stromal cells. Design Prospective study. Setting Department of Ob/Gyn, Tottori University, Japan. Patient(s) Thirty-five patients with ovarian endometrioma and ten patients with uterine myoma. Intervention(s) Endometriotic stromal cells were obtained from chocolate cyst linings of ovaries. Endometrial stromal cells obtained from patient with uterine myoma. Main Outcome Measure(s) Expression of IL-10 gene in endometriotic or endometrial stromal cells was determined by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). We performed immunohistchemical staining to find the presence of IL-10 and IL-10 receptors 1 and 2. We examined the effects of TNF-α and IL-10 on the expression of IL-6 or IL-8 by real-time RT-PCR and ELISA. We examined the activation of intracellular signal transduction molecules in endometriotic stromal cells by Western blotting. Result(s) Addition of IL-10 suppressed the expressions of IL-6 induced by TNF-α and IL-10 induced the phosporylation of STAT3 in endometriotic stromal cells. TNF-α induced the expression of phosphorylated ERK1/2, JNK1/2, and IκB. Adding IL-10 suppressed the phosphorylation of these signal molecules. Conclusion(s) Interleukin-10 attenuates TNF-α–induced IL-6 synthesis via NF-κB and MAPK pathways in endometriotic cells.. Interleukin-10 may play a significant role in the pathogenesis of endometriosis.