Abstract: Endometriosis, defined by the presence of viable endometrial tissue outside the uterine cavity, is among the most common gynecologic disorders affecting women of reproductive age. Endometriosis is associated with an inflammatory peritoneal environment, where multiple cytokines and growth factors are found at elevated levels. Interleukin-8 (IL-8) is a cytokine that induces chemotaxis of neutrophils and is a potent angiogenic agent. In addition, IL-8 was recently found to stimulate proliferation of various cells. We have observed that IL-8 is elevated in the peritoneal fluid of women with endometriosis and the levels correlate with the severity of the disease. We hypothesized that IL-8 may play a role in the growth and maintenance of ectopic endometrial tissue not only by chemoattracting and stimulating leukocytes to secrete growth factors and cytokines, but also by directly affecting endometrial cell proliferation. We found that IL-8 mRNA and protein levels in the endometrium were significantly higher during early proliferative and late secretory phases than during the mid-cycle. IL-8 receptors A and B are also expressed in the endometrium mostly localized in the stroma. Interestingly, IL-8 receptor expression is higher in the eutopic endometrium of women with endometriosis compared to the endometrium of women without endometriosis. Endometrial cells in culture proliferate significantly when treated with IL-8, which is inhibited by anti-IL-8 neutralizing antibody. More convincingly, IL-8 antisense oligonucleotide treatment decreases IL-8 production by endometrial cells as well as cell proliferation when compared to non-sense oligonucleotide treatment. The addition of IL-8 reverses the inhibitory effect of IL-8 antisense oligonucleotides on cell proliferation. These findings suggest that IL-8 may act as an autocrine growth factor in the endometrium. We have also studied the effect of endometrial cell adhesion on IL-8 expression and observed that IL-8 stimulates the adhesion of endometrial cells to fibronectin. Treatment of the cells with anti-IL-8 neutralizing antibody inhibited partially the cell adhesion. Thus, IL-8 may also be relevant for stimulating the attachment of endometrial implants in the pathogenesis of endometriosis. In addition, adherence of endometrial cells induced further IL-8 expression by an integrin-dependent mechanism. In summary, IL-8 may act as an autocrine growth factor in the endometrium and may also play a role in the pathogenesis of endometriosis by promoting the vicious circle of endometrial cell attachment, cell growth, and further secretion of this cytokine.