Atopic dermatitis (AD) is an inflammatory skin disease associated with increased systemic and vascular inflammation. Although dupilumab has been proven to be effective against severe AD, imaging studies analysing its inflammation-reducing effect have rarely been reported. The aim of this study was to evaluate the effect of dupilumab on systemic and vascular inflammation in adult patients with severe AD, using 18F-fluorodeoxyglucose positron emission tomography-computed tomography (18F-FDG PET/CT). A total of 33 adult patients with severe AD and 25 healthy controls underwent 18F-FDG PET/CT at baseline. Patients on dupilumab treatment underwent 18F-FDG PET/CT again after achieving a 75% reduction from baseline on the Eczema Area and Severity Index (EASI-75). Patients with AD exhibited increased 18F-FDG uptake values in the liver, spleen, pancreas, and carotid artery compared with healthy controls. However, compared with baseline, there was no statistically significant difference in 18F-FDG uptake in major organs and arteries after achieving EASI-75 with dupilumab treatment. In conclusion, while dupilumab treatment resulted in a significant clinical improvement and reduced serum inflammatory markers in adult patients with severe AD, no changes in systemic and vascular inflammation were observed on 18F-FDG PET/CT imaging.