OBJECTIVE To investigate the association between concentrations of soluble intercellular adhesion molecule-1 (ICAM-1) and interferon-gamma (IFN-gamma) with regard to the severity of endometriosis. METHODS Cross-sectional study. METHODS Infertility unit in a university hospital. METHODS Seventy-one infertile patients who underwent laparoscopic evaluation were divided into two groups: 36 patients with endometriosis served as cases, and 35 patients without endometriosis served as control subjects. METHODS Peripheral blood and peritoneal fluid (PF) were collected from these patients during laparoscopic examination. METHODS Quantitative determination of levels of human ICAM-1 and IFN-gamma was performed with the use of an ELISA. The data were evaluated by Student's t-test, analysis of variance, and regression analysis. RESULTS Significantly elevated levels of soluble ICAM-1 were found in the sera of patients with endometriosis, especially those with advanced stages of the disease. Decreased IFN-gamma levels, which were negatively correlated with serum levels of soluble ICAM-1, also were noted in the PF of patients with endometriosis. CONCLUSIONS The increased serum levels of ICAM-1 found in patients with endometriosis may indicate the presence of an active disease process. Further, the increased levels of soluble ICAM-1 in peripheral blood were inversely correlated with the concentrations of IFN-gamma in PF. The increased levels of soluble ICAM-1 in plasma may be associated with an immunologic feedback response that blocks further infiltration of immune cells. These findings may be of value in the diagnosis and evaluation of endometriosis.