The aim of the study was to investigate the clinical value of the serum CA 125 level for diagnosing and determining the severity of endometriosis and pelvic pain associated with endometriosis. Eighty-six women who underwent operative laparoscopy were enrolled. Sixty-nine women with endometriosis and 17 without endometriosis participated in this study. In all of the patients, endometriosis was diagnosed and classified into stages according to the Revised American Fertility Society (R-AFS) classification. The mean serum CA 125 levels were determined in each patient. We also investigated the relationship between serum CA 125 concentration and the intensity of dysmenorrhea and dyspareunia in the study group. The mean serum CA 125 levels of women with endometriosis were higher than those of the control group (p<0.050). However, the mean serum CA 125 levels were higher in stage IV than in other stages of endometriosis according to the R-AFS classification. On the other hand, the percentage of patients with serum CA 125 levels >35 U/mL was elevated in the subgroups with severe dyspareunia and severe dysmenorrhea versus the asymptomatic subgroup but the differences had no statistical significance. In conclusion, CA 125 serum levels were related to endometriosis and R-AFS score in the evaluated patient series. No correlation was found between serum levels of CA 125 and pelvic pain in patients with endometriosis.