Endometriosis is a gynaecological disease with a certain genetic background, but the locations of possible genomic aberrations are still poorly clarified. Intercellular adhesion molecule-1 (ICAM-1), which is a surface glycoprotein that promotes adhesion in immunological and inflammatory reactions, seems to play a role in this condition. The aim of this study was to examine the potential associations of ICAM-1 gene polymorphisms with endometriosis and its severity. Specifically, we have studied two polymorphic sites located in codons 241 (G/R241) and 469 (E/K469) of the ICAM-1 gene. Three hundred and sixty-three Italian Caucasian women of reproductive age who underwent laparoscopy for benign pelvic conditions were enrolled in the study. Endometriosis was documented and staged in 188 women while 175 subjects, in whom endometriosis was laparoscopically ruled out, served as the control group. The frequency of the R241 allele was only marginally higher in endometriosis patients than in controls [5.8 versus 2.9%, P = 0.05; odds ratio (OR), 2.1; 95% confidence interval (CI), 1-4.5]. However, a strikingly high frequency of this allele was found in patients with Stage IV endometriosis versus controls (8.6 versus 2.8%, P = 0.008; OR, 3.2; 95% CI, 1.3-7.9). In contrast, the allele and genotype frequencies of the E/K469 polymorphism did not differ significantly between endometriosis and control groups. While the functional correlate of the G/R241 polymorphism remains unclear, this finding indicates that a genetic polymorphism in the ICAM-1 gene domain may contribute to the susceptibility to endometriosis.