Purpose of investigation: To assess the changes secondary to chronic inflammation in women with and without pelvic endometriosis by the determination of serum thiols and carbonyls. Materials and Methods: Sixty-seven women with endometriosis consecutively submitted to laparoscopy and 41 women without endometriosis consecutively submitted to tubal ligation (control group) were selected. Serum levels of total thiols and carbonyls were determined in both groups. Results: Patients with endometriosis had significantly lower thiol levels than controls (342.37 ± 142.09 μM vs 559.60 ± 294.05 μM) (p < 0.001), as well as significantly lower carbonyl levels (8.97 ± 3.76 μM vs 16.40 ± 9.26 μM) (p < 0.001). Other clinical characteristics were not associated with changes in marker levels. The cutoff point established by the ROC curve was 396.44 μM for the thiols, with 73.1% sensitivity and 80.5% specificity, and 14.9 μM for the carbonyls, with 94% sensitivity and 51.2% specificity. Conclusions: The serum thiol levels revealed an increase in oxidative stress related to the development of pelvic endometriosis.