Adrenocortical carcinoma (ACC) is a rare type of cancer that is associated with a high rate of recurrence and poor prognosis. The main diagnostic approaches to adrenocortical cancer include CT scan, MRI and the promising role of 18F-FDG PET/CT. The main therapeutic approaches include radical surgery of local disease and recurrences, as well as adjuvant mitotane therapy. The evaluation of adrenocortical carcinoma (ACC) could be difficult by using 18F-FDG PET/CT in view of the significant association between the 18F-FDG uptake and ACC. At the same time, not all adrenal glands with 18F-FDG uptake are considered to be malignant, so awareness of these various findings is substantial for ACC management, especially with limited data regarding the role of 18F-FDG PET/CT in ACC post-operative settings. This report discusses the case of a 47-year-old man with a history of left adrenocortical carcinoma who underwent adrenalectomy and received adjuvant mitotane therapy. 9 months after the surgery, a follow-up 18F-FDG PET/CT scan showed that the 18F-FDG uptake was prominent in the right adrenal gland without corresponding abnormal CT scan findings.