A 67-year-old woman with prolonged fever, thrombocytopenia, and renal dysfunction underwent FDG PET/CT to evaluate underlying causes, including malignancy. PET/CT showed FDG uptake in ascites, subcutaneous edema, lymph nodes, spleen, and bone marrow. Subsequent bone marrow biopsy revealed myelofibrosis, and laboratory testing showed elevated concentrations of interleukin 6 in serum and ascites. These findings led to the diagnosis of TAFRO syndrome, a variant of multicentric Castleman disease, characterized by thrombocytopenia, anasarca, fever, reticulin myelofibrosis, and organomegaly. Because TAFRO syndrome is potentially fatal, accurate diagnosis is crucial. Characteristic FDG PET/CT findings facilitate the diagnosis of TAFRO syndrome, which is generally challenging.