To evaluate the clinical utility of CA-125 in the diagnosis of endometriosis and to compare the sensitivity of the serum and the peritoneal test as indicators of disease, the quantitative determination of the antigen in serum and in peritoneal fluid was performed by the IRMA-mat "two-step method", a two-site immunoradiometric assay. A total of 28 women undergoing diagnostic or operative laparoscopy with endometriosis stage I-II (10 patients), endometriosis stage III-IV (8 patients) or other benign gynecological diseases (10 patients) were studied. The results were compared with a sample of 12 women with a normal pelvis (control group). CA-125 levels in the peritoneal fluid were higher than those found in the serum and were significantly elevated (p < 0.001) when compared with the control group, both in women with endometriosis stage I-II and stage III-IV. In the serum CA-125 levels increased only in advanced stages of endometriosis. Levels of CA-125 in peritoneal fluid seemed to be a more sensitive indicator of disease than the serum test (0.88 vs 0.44), especially in early stage endometriosis (0.80 vs 0.20) which tends to be overlooked by the CA-125 serum test.