Background: Endometriosis affects at least 15% of infertile women. The diagnosis is made by direct visualization of implants either at laparoscopy or laparotomy. Elevation of plasma CA-125 levels has been associated with the presence of endometriosis. Aim: To search for a correlation between CA-125 and the presence of endometriosis and its severity, among infertile patients requesting a diagnostic laparoscopy. Patients and methods: A total of 117 women were prospectively enrolled in the study between May 2000 and March 2002. A serum sample for CA-125 was taken immediately before surgery and the value obtained was correlated with the presence or absence of endometriosis and the severity of the disease. Results: Sixty nine patients had endometriosis (59%). Patients with severe endometriosis, or grade IV, showed statistically significant higher levels of CA-125 compared to women without endometriosis or with grades I, II or III of the disease. Discussion: The overall sensibility of CA-125 in detecting patients with endometriosis was low, however, it worked better for patients with advanced disease. Considering the rather low cost and simplicity of the test, we believe that it should be included in the evaluation of infertile couples (Rev Méd Chile 2003; 131: 367-72).