Objective: To compare the expression of class I human leukocyte antigen (HLA I) in endometrial samples from patients with and with out endometriosis. Design: Cross- sectional study. Setting: Acute-care teaching hospital in Barcelona, Spain. Patient(s): The endometriosis group included 32 patients for whom the only diagnosis during an operation was endometriosis. The control group included 20 women who underwent a laparoscopy and in whom no evidence of endometriosis or any other genital disease was seen. Intervention(s): Samples of endometrium were obtained by curettage and immediately frozen. A pan-HLA I mouse antihuman IgG2a monoclonal antibody was used for immunohistochemical study. Main Outcome Measure(s): Frequency of positive glandular and stromal cells was evaluated in each section. Result(s): A significantly higher expression of HLA I in the endometriosis group than in controls, both in the glandular cells (median 100%vs 80%) and in the stromal cells (median 60%vs 20%), was observed. Conclusion(s): Patients with endometriosis had a significantly higher expression of HLA I molecules in endometrial cells than did the controls. This could be a possible explanation for their higher resistance to natural killer cytolysis. Objective: To compare the expression of class I human leukocyte antigen (HLA I) in endometrial samples from patients with and with out endometriosis. Design: Cross- sectional study. Setting: Acute-care teaching hospital in Barcelona, ​​Spain. Patient (s) The control group included 20 women who underwent a laparoscopy and in whom no evidence of endometriosis or any other genital disease was seen. Intervention (s): Samples of Amber-HLA I mouse antihuman IgG2a monoclonal antibody was used for immunohistochemical study. Main Outcome Measure (s): Frequency of positive glandular and stromal cells were as in each section. Result (s): A significantly higher expression of HLA I in the endometriosis group than in controls, both in the glandular cells (median 100% vs 80%) and in the stromal cells (median 60% vs 20%), was obse Conclusion (s): Patients with endometriosis had a significantly higher expression of HLA I molecules in endometrial cells than did the controls. This could be a possible explanation for their higher resistance to natural killer cytolysis.