e11561 Background: Chemotherapy-induced oxidative stress (CIOS) can be altered multifactorially in time course of treatment. Initial changes of CIOS in the breast cancer patients are not clear. OBJECTIVES: IRB-approved prospective study of the changes of CIOS in chemo-naïve and radiotherapy-naïve breast cancer patients who undergo chemotherapy(CT) or radiotherapy(RT) in either adjuvant or metastatic settings. Methods: Exclusion criteria: Patient who has prior exposure to CT or RT, active infection, connective tissue disorder, been taking antioxidant agent(s), active hepatitis, liver cirrhosis, renal insufficiency and no written informed consent. Blood samplings were done before CT or RT and day 22 since chemotherapy or day 29 since RT. Serum analyzed for glutathione peroxidase(GSH-Px), glutathione reductase(GR), Vit.E, malondialdehyde (MDA), and trace elements including Zn, Cu, Fe, Se. Results: 10 patients and 10 healthy volunteers were entered. Median age: 47.5 (R: 38~80); Tumor type; infiltrating ductal ca: 8 pts; lobular ca: 1; mucinous: 1. Stage: IIA: 4 pts; IIB: 1; IIIA: 3; IIIB: 0; IIIC: 1; Relapsed: 1. ECOG 0/1: 9/1 pts; Regimens: FEC: 5 pts, CAF: 1, EC: 2, CMF: 1; RT: 1. After 1 cycle of CT or 28 days of RT: GSH-Px was significantly increased (P=0.0003); GR increased (P=0.0002); Vit. E increased (1-sided P=0.028); MDA decreased (P=0.0073) and Zn decreased (P=0.0162). Contrary to some oncologists initial expectations, the early changes after chemotherapy or radiotherapy are elevation of body's antioxidative capacity. Conclusions: Spontaneous counterbalance of CIOS after initial cycles of chemotherapy is suggested from this study.