18F-fluoro-deoxy-glucose positron emission tomography/computed tomography (FDG PET/CT) is commonly used in head and neck cancer staging. This investigation frequently identifies incidental findings unrelated to the index head and neck cancer. To assess the nature of these incidental findings, notes of patients who underwent FDG PET/CT with primary head and neck cancer between January 2014 and June 2015 at the University of Wisconsin were retrospectively reviewed. At their institution, two reports were generated for each FDG PET/CT – one of the PET, another of the CT. There were 293 patients included. Of these, 35.2% (103) had incidental findings, 24 having multiple. There were 174 scans performed post-treatment and 119 pre-treatment. There was no significant difference between incidentals in scans post- or pre-treatment (p = 0.093). Of 134 total incidental findings, 47 (35.1%) were radiologically potentially malignant. Twelve (25.5%) of these suspicious findings were ultimately malignant. Therefore, the rate of malignant incidental findings was 9% (12 of 134 incidental findings) – five thyroid, two lung, two gastrointestinal, one head and neck, one lymphoma and one genitourinary. Incidental findings irrespective of radiological suspicion of malignancy were 45 (33.6%) in the head and neck (most of dental/sinus disease), 29 (21.6%) gastrointestinal, 15 (11.2%) thyroid, 12 (9%) lungs, 11 (8.2%) genitourinary, eight (6%) skeletal, and 14 (20.4%) elsewhere. Fifty-four (40.3%) of all incidental findings were identified on PET, 33 (24.6%) on non-contrast CT and 21(15.7%) CT with contrast. Of potentially malignant findings, 13 (27.7%) were on PET, eight (17%) on CT with contrast and five (10.6%) on non-contrast CT. Of possible malignant findings, 12 had no specific recommendations, 10 were recommended for further imaging, seven biopsy, four colonoscopy / oesophagoscopy, one parotid biopsy, one renal ultrasound, and 12 follow-up or physical examination. This study shows that a significant number of incidental findings on FDG PET/CT are malignant and therefore when detected require additional evaluation.