Posttreatment follow-up for the recurrence of head and neck squamous cell carcinoma (HNSCC) is a diagnostic challenge. Tissue distortion from radiation and surgery can obscure early detection of recurrence by conventional follow-up approaches such as physical examination or conventional imaging. Fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT is widely validated for the diagnosis of suspected recurrence. Moreover, we have shown in a previous prospective study the high effectiveness of FDG PET/CT in the assessment of subclinical recurrence 12 months after treatment. The aim of this prospective study was to evaluate the effectiveness of an earlier FDG PET/CT, at 6 months after the end of treatment. All patients treated for histologically proven HNSCC from April 2009 to May 2012 at the University Hospital of Brest who did not show any findings suggestive of recurrence at 6 months of their usual follow-up underwent an FDG PET/CT examination. FDG PET/CT findings were correlated with histopathology or imaging follow-up. The study included 116 patients. FDG PET/CT examinations were performed within a mean period ± SD of 5.6 ± 1.8 months after treatment. FDG PET/CT examinations exhibited abnormal FDG uptake in 34 patients and found no suspected recurrence in 82 cases. Of these 82 FDG PET/CT considered as negative, only 1 had a recurrence. Among the 34 positive FDG PET/CT, 22 relapsed whereas 12 did not show evidence of recurrence. The sensitivity and specificity of FDG PET/CT in this study for the diagnosis of occult HNSCC recurrence were 96 (22/23) and 87 % (81/93), respectively. The positive predictive value was 65 % (22/34). The negative predictive value was 99 % (81/82). The overall accuracy was 89 % (103/116). Of the 116 patients, FDG PET/CT highlighted 22 (19 %) subclinical recurrences. Our study showed the high effectiveness of FDG PET/CT in the assessment of subclinical HNSCC recurrence 6 months after completion of treatment. These results confirmed that FDG PET/CT is more accurate than conventional follow-up physical examination alone in the assessment of recurrence after previous curative treatment for HNSCC, as we previously demonstrated in patients clinically asymptomatic at 12 months.