We report a case of a 38-year-old man affected by the human immunodeficiency virus (HIV) with a diagnosis of Castleman's disease, plasmablastic type human herpes virus 8 infection, and Kaposi sarcoma based on a histological examination of one cervical lymph node biopsy. The patient underwent (18)F-2-fluoro-2-deoxyglucose positron emission tomography/computed tomography ((18)F-FDG-PET/CT). (18)F-FDG-PET/ CT seems to be a valuable tool in patients with HIV-associated Castleman's disease and Kaposi sarcoma. It allows accurate staging and identifies more sites of disease than conventional CT.