The relationship between the immune and the endometrial systems has been recently suggested to be critical to the development of endometriosis. We previously showed that one of the molecules involved in the complex events that allow this interaction is the adhesion molecule intercellular adhesion molecule (ICAM)-1. This study was designed to evaluate whether differences in ICAM-1 mRNA and protein expression might exist between eutopic endometrial cells and ectopic cells derived from endometriomas. Stromal cells were dispersed from samples of endometrium and ovarian endometriomas biopsied synchronously from 24 patients with endometriosis. We established that the relative expression of ICAM transcript was significantly higher in ectopic cells than that found in cultures derived from endometrial samples. Moreover, ectopic cells demonstrated a significant overexpression of ICAM-1 protein in both its cell-bound and soluble form (sICAM-1) (P < 0.05). Interestingly, endometrial secretion of sICAM-1 was shown to vary during the menstrual cycle as proliferative phase samples released significantly higher concentrations of the soluble protein compared to the secretory phase. In contrast, this cycle-dependent pattern was absent in stromal cells derived from endometriomas. Moreover, interleukin (IL-1beta) was able to increase sICAM-1 shedding from endometrial cells in a concentration-dependent manner and this IL-1beta-mediated induction could be slightly enhanced by oestradiol. As sICAM-1 is able to interfere with ICAM-1-mediated immune functions, the release of higher concentrations from ectopic samples may be the mechanism by which ectopic endometrial cells escape immunosurveillance.