Background: Radiation-induced fatigue (RIF) is common in breast cancer (BC) survivors and can last years after treatment. Despite the known physiological consequences of radiation, information on biomarkers of RIF is limited. Therefore, we examined the longitudinal association between serum biomarkers and post-BC fatigue in BC survivors treated with radiation: [oxidative stress] 8-hydroxyguanosine (8-OH-dG), myeloperoxidase (MPO); [inflammation] interleukin-6 (IL-6), C-reactive protein (CRP), growth differentiation factor-15 (GDF-15), placental growth factor (PGF), transforming growth factor-beta (TGF-B); [cardiac damage] cystatin-C, troponin-I (TnI). Methods: In an ancillary study in the Women’s Health Initiative (WHI), participants with incident BC (stages I-III) treated with radiation were followed for the development of fatigue post-BC. Women were eligible if they 1) had no prior cardiovascular disease and 2) had pre-and post-BC serum samples drawn approximately three years apart with fatigue measured using the Short-Form 36 (SF-36) vitality subscale at the same times. Biomarkers were analyzed using enzyme-linked immunosorbent assays. Higher SF-36 vitality scores correspond to lesser fatigue. Weighted linear regression adjusted for relevant demographic, lifestyle, and psychosocial factors, as well as pre-cancer fatigue. Each biomarker was modeled as the post-BC to pre-BC ratio and log transformed to base 2, thus, the effect estimates correspond to a doubling in value compared to pre-BC. Results: A total of 180 women with a mean (SD) age of 67.0 (5.5) were included. The mean (SD) vitality score was 66.2 (17.2) and 59.7 (19.7) pre- and post-BC, respectively. The median (IQR) time between pre-BC serum collection to BC was 1.9 (0.8, 2.6) years and 1.4 (0.7, 2.3) years between BC and the post-BC serum collection. After adjustment, a higher biomarker ratio of cystatin-C, IL-6, and GDF-15 were all associated with a lower SF-36 vitality score (i.e., higher fatigue) (Table). As an example, for a 2-fold difference in the cystatin-C biomarker ratio, the SF-36 vitality score was lower by 7.31 points (95% CI: -14.2, -0.45). Conclusions: Inflammatory and cardiac damage biomarkers are associated with RIF in BC survivors. Biomarkers could be measured in clinical practice or be included in risk prediction models to help identify patients at high risk for RIF.