Background: Fatigue is one of the most debilitating adverse effects of cancer therapy. Identifying biomarkers early during cancer therapy may help us understand the biologic underpinnings of the persistence of fatigue following therapy. Objective: We aimed to identify early biomarkers of fatigue by examining correlations of levels of cytokines during external beam radiation therapy (EBRT) with persistence of fatigue 1 year following treatment completion in men with nonmetastatic prostate cancer (NM-PC). Methods: A sample of 34 men with nonmetastatic prostate cancer scheduled to receive EBRT were followed up at baseline (T1), midpoint of EBRT (T2), and 1 year following EBRT (T3). Demographic and clinical data were obtained by chart review. The Functional Assessment of Cancer TherapyYFatigue was administered to measure fatigue levels. Plasma cytokine levels were determined at T1 and T2 using the Bio-Rad Bio-Plex Cytokine Assay Kits. Results: Significant correlations were observed between levels of interleukin 2 (IL-3), IL-8, IL-9, IL-10, IL-16, interferon + Yinduced protein 10, interferon alpha 2, interferon +, and stromal cell-derived factor 1 alpha at T2 with worsening of fatigue from T1 to T3. Conclusions: Immunological changes prior to chronic fatigue development may reflect the long-term response to radiation therapyYinduced damage. Implications for Practice: Early biomarkers for chronic fatigue related to cancer therapy will help advance our understanding of the etiology of this distressing symptom and will help nurses identify patients at risk of developing chronic fatigue after cancer treatment. This information will also aid in patient education, as well as symptom management.