Microscopic polyangiits (MPA) is an autoimmune vasculitis that is challenging to diagnose because it can present with myriad of manifestations. We present a case of a woman who presented with prolonged fever. Her diagnosis of MPA was realized after fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (F-18 FDG-PET/CT) showed abnormal diffuse renal uptake in the cortex, leading to a kidney biopsy. Our report supported the value of FDG-PET/CT as a diagnostic tool in a patient with febrile illness without localizing symptoms or signs.