Background and Clinical Significance: Fever of undetermined origin (FUO is a diagnostic challenge. It is essential to exclude infections, paraneoplastic syndromes, and large-vessel vasculitis (LVV). Case presentation: We describe a 59-year-old female with FUO and no apparent signs of infection. Laboratory results were consistent with inflammation (ESR 83, CRP 203 (ref. value (RV) < 5 mg/dl), ferritin 311 (RV < 120 µg/L), microcytic anemia, thrombocytosis. With administration of both ceftriaxone and levofloxacin, a decrease in CRP was monitored (51 mg/L). HRCT of the chest, abdomen, and pelvis revealed a saccular aneurysm of the descending thoracic aorta and an ectatic right common iliac artery. Due to suspicion of LVV, CT angiography was performed to exclude inflammatory changes in the blood vessels. Diffuse atherosclerosis with a 30 mm penetrating thoracic aortic ulcer (PAU) was found. PET-CT and leukocyte scintigraphy were used to rule out vasculitis and infection. The patient was successfully treated with Thoracic Endovascular Aortic Repair (TEVAR). Conclusions: As sophisticated imaging techniques become more widely used, more PAUs are being detected as incidental abnormalities in individuals without acute aortic syndrome. With adequate management, many people with PAU can live a stable and healthy life without experiencing significant consequences.