Background: 18 F-fluoro-2-deoxy-D-glucose ( 18 F-FDG) accumulations are commonly seen in the neck-related muscles of the surgical and non-surgical sides after surgery with neck dissection (ND) for oral cancers, which leads to radiologists having difficulty in diagnosing the lesions. To examine the alterations in 18 F-FDG accumulation in neck-related muscles of patients after ND for oral cancer. Material and Methods: 18 F-FDG accumulations on positron emission tomography (PET)-computed tomography (CT) in neck-related muscles were retrospectively analyzed after surgical dissection of cervical lymph nodes in oral cancers. Results: According to the extent of ND of cervical lymph nodes, the rate of patients with 18 F-FDG-PET-positive areas increased in the trapezius, sternocleidomastoid, and posterior neck muscles of the surgical and/or non-surgical sides. In addition, SUVmax of 18 F-FDG-PET-positive areas in the trapezius and sternocleidomastoid muscles were increased according to the extent of the ND. Conclusions: In evaluating 18 F-FDG accumulations after ND for oral cancers, we should pay attention to the 18 F-FDG distributions in neck-related muscles including the non-surgical side as false-positive findings