We report the case of a 72 year old man with clinical suspicion of giant cell arteritis (GCA); in this case 18F-FDG PET/CT has been useful from the diagnosis to the control of the response to therapy. He performed the first 18F-FDG PET/CT that showed increased uptake of 18F-FDG in the aortic arch, brachial artery, common iliac and femoral arteries and in the temporal cerebral sites bilaterally. The patient then began a cortisonic therapy that led straight to the improvement of clinical symptoms, and repeated 18F-FDG PET/CT 3 months later the beginning of therapy that showed reduced uptake in the same sites of the first 18F-FDG PET/CT. This patient continued the cortisonic therapy, progressively reducing it until the definitive ending.