Viral infections, including cytomegalovirus (CMV) and Epstein-Barr virus (EBV), play an important role in carcinogenesis and can influence patients' prognosis and condition during cancer treatment.The goal of this study was to investigate CMV and EBV infections in patients receiving radiotherapy or radiochemotherapy due to head and neck cancers to determine the influence of these infections on the risk of death. The observation period was 2 years.Of 41 patients enrolled, 11 received radiotherapy (simultaneous-integrated boost intensity-modulated radiation therapy [SIB-IMRT], 2.25 Gy/fraction, 30 fractions, [n = 7] or IMRT, 2 Gy per fraction, 35 fractions, [n = 4]) and 30 received radiochemotherapy (cisplatin 100 mg/m and SIB-IMRT [n = 13] or IMRT [n = 17]). Plasma CMV and EBV DNA levels were assessed using real-time PCR before or during treatment or 4 weeks posttreatment.The risk of death in the group positive for plasma CMV or EBV deoxyribonucleic acid (DNA) was significantly higher compared to the group without detectable plasma CMV (odd ratio [OR]: 7.5, 95% confidence interval [CI]: 1.11-50.67) or EBV DNA (OR: 10.91, 95% CI: 1.135-104.8). Results were confirmed using the Bayesian method. Plasma positivity for CMV or EBV DNA was associated with a higher risk of death (both P = .04).Viral infections negatively affect the survival of patients with head and neck cancers. Diagnosing and treating these viral infections in patients with positive results should be considered.