The purpose of this study was to compare whole-body fluorine-18-fluorodeoxyglucose positron emission tomography (F-18 FDG-PET) with conventional imaging modalities (CI: CT/MRI) in the detection of recurrent head and neck cancer. Whole-body F-18 FDG-PET was performed in 45 patients (recurrence = 39; no recurrence = 16) with previous head and neck cancer. We compared detectability by the period from initial cancer treatment and treatment modalities. Thirty were PET-positive and 15 were PET-negative. The sensitivity, specificity, and accuracy of PET were 97%, 88%, and 93%, respectively (corresponding figures of CI were 73%, 85%, and 77%). In 18 patients who underwent PET less than 3 months after the completion of cancer treatment, the sensitivity, specificity, and accuracy were 100%, 86%, and 94%, while for CI, the corresponding figures were 67%, 71%, and 69%. In 18 patients who had undergone surgery, PET results were 14 true positive and 4 were true negative; significantly higher detectability than CI. Among the patients who were evaluated for more than 6 months or treated by radiotherapy without surgery, diagnostic accuracy was almost the same. Whole body F-18 FDG-PET was a valuable tool in the evaluation of post-therapeutic recurrence of head and neck cancer.