The incidence of malignant adrenal tumors, including primary adrenal lymphoma (PAL) and adrenocortical carcinoma (ACC), is rather low. Early differentiation between ACC and PAL is necessary because the therapeutic strategies for the two differ and the prognosis of each disease is poor. Unfortunately, there is no pathognomonic characteristic of PAL, so radiologists have difficulty in differentiating it from ACC. To our knowledge, there have been only two case reports on ACC that was evaluated and followed up using F-18 FDG PET/CT. The previously reported cases were simple, involving only the bilateral adrenal glands. Here, we report on a 52-year-old man diagnosed with PAL that radiologically mimicked ACC. He had findings of heterogeneous enhancement on CT, several adjacent lymphadenopathies with internal necrosis, and abutting hepatic and bilateral crural invasion. After pathological confirmation, we monitored his prognosis using F-18 FDG PET/CT after three cycles of rituximab-CHOP and again after six cycles of treatment.