A 29-year-old woman underwent resection of a left anterior mediastinal thymoma and pleurectomy. Postsurgical 18FDG PET/CT scan demonstrated FDG avidity in the right neck and upper thoracic fat but relatively absent FDG-avid fat in the left neck and upper thorax. Bilateral FDG-avid fat was also apparent in the lower chest and upper abdomen. After surgery, the patient demonstrated Horner syndrome, with left-sided ptosis, miosis, and facial anhidrosis. It is hypothesized that left-sided sympathetic nerves were compromised during surgery, leading to Horner syndrome and denervation of ipsilateral brown fat. The unilateral FDG avidity should not be mistaken for malignancy.