Giant cell arteritis (GCA) is a systemic vasculitis which typically occurs in persons over 50 years old. GCA is closely related to polymyalgia rheumatica (PMR). A temporal artery biopsy is the gold standard test for the diagnosis of GCA. Recently, there is increasing evidence for the role of 18F-fluorodeoxyglucose positron emission tomography (18F-FDG-PET) in diagnosis of vasculitis. Here, we report on a case of a 67-year-old Korean male who was diagnosed with atypical GCA in subclinical stage concomitant with PMR by 18F-FDG-PET. After treatment, abnormal findings of 18F-FDG-PET were improved.