We previously suggested that women with endometriosis have increased oxidative stress in the peritoneal cavity. To assess whether antioxidant supplementation would ameliorate endometriosis-associated symptoms, we performed a randomized, placebo-controlled trial of antioxidant vitamins (vitamins E and C) in women with pelvic pain and endometriosis. Fifty-nine women, ages 19 to 41 years, with pelvic pain and history of endometriosis or infertility were recruited for this study. Patients were randomly assigned to 2 groups: vitamin E (1200 IU) and vitamin C (1000 mg) combination or placebo daily for 8 weeks before surgery. Pain scales were administered at baseline and biweekly. Inflammatory markers were measured in the peritoneal fluid obtained from both groups of patients at the end of therapy. Our results indicated that after treatment with antioxidants, chronic pain (“everyday pain”) improved in 43% of patients in the antioxidant treatment group (P= 0.0055) compared with the placebo group. In the same group, dysmenorrhea (“pain associated with menstruation”) and dyspareunia (“pain with sex”) decreased in 37% and 24% patients, respectively. In the placebo group, dysmenorrhea-associated pain decreased in 4 patients and no change was seen in chronic pain or dyspareunia. There was a significant decrease in peritoneal fluid inflammatory markers, regulated upon activation, normal T-cell expressed and secreted (P≤ 0.002), interleukin-6 (P≤ 0.056), and monocyte chemotactic protein-1 (P≤ 0.016) after antioxidant therapy compared with patients not taking antioxidants. The results of this clinical trial show that administration of antioxidants reduces chronic pelvic pain in women with endometriosis and inflammatory markers in the peritoneal fluid.