Giant cell arteritis (GCA) is easily diagnosed in case of typical symptoms such as headache and jaw claudication. We present an 82-year-old man who was admitted due to symptoms of pneumonia, weight loss and fever. He did not respond to antibiotic treatment. An 18F-fluordeoxyglucose PET-CT scan showed increased uptake of the thoracic branches of aorta, raising the suspicion of GCA. The result from a subsequent temporal artery biopsy supported the diagnosis. Based on the clinical symptoms, the results from the PET-CT scan and the biopsy, and the response to high dose of prednisolone, the patient was diagnosed with GCA.