A 29-year-old man with Behçet disease exhibiting lower-limb swelling and dyspnea underwent 18F-FDG PET/CT. The imaging revealed bilateral pulmonary artery involvement, and pulmonary artery aneurysms and mural thrombus were confirmed by CT pulmonary angiography. This case underscores the utility of 18F-FDG PET in identifying life-threatening vasculitis complications in Behçet disease.