Background/Aim: F-18-misonidazole positron emission tomography (FMISO PET)/computed tomography (CT) obtained before and during radiotherapy (RT) was analyzed as to whether it could predict clinical outcome. Patients and Methods: Twenty-two patients were included. FMISO PET/CT was performed twice before RT and at a dose of approximately 20 Gy/10 fractions. FMISO maximum standardized uptake values (SUVmax), the tumor-to-muscle ratios (T/M), and hypoxic volume (HV) in gross target volumes were measured. Results: Of the 22 tumors, 18 had hypoxic areas (SUVmax = 1.60) before RT. SUVmax, T/M, and HV on the first PET/CT were significantly correlated with initial tumor response, although the values during RT were not related to the response. The overall survival and locoregional control rates of patients below cut-off values were significantly better than those above the cut-off values. Conclusion: Tumor hypoxia detected by FMISO PET/CT before RT may predict clinical outcome.