The fat-rich anterior mediastinum could be a sensitive window for monitoring minute changes in vascularity induced by systemic vasculitis. To evaluate this hypothesis, an analysis of anterior mediastinal fat in patients with Behçet's disease and a control group was conducted. This study included 43 patients diagnosed with Behçet's disease within the last 11 years who underwent CT scan; 55 patients were selected as a control population. Mediastinal fat was classified according to CT morphology. Comparison of serum inflammatory markers was performed for evaluation of disease activity according to morphologic types, and average Hounsfield unit of the anterior mediastinum was measured. Significantly higher mean CT attenuation was observed in the Behçet's disease group, compared with the control group (-48.5 ± 33.5 vs. -67.7 ± 18.7, respectively, P < 0.05). Mediastinal fat types were classified as follows: pure fatty tissue (2 vs. 31 % [Behçet's disease vs. control group]), diffuse soft tissue infiltration (16 vs. 29 %), tubular structures (21 vs. 4 %), mixed infiltration with tubular structures (42 vs. 15 %), and evident thymic tissue (19 vs. 22 %). The value for mean mediastinal attenuation was significantly higher in the group with a high level of C-reactive protein than in the normal level group. The mean CT attenuation of anterior mediastinal fat is significantly higher in the Behçet's disease group, compared with the normal group. Although pathologic confirmation is needed, the cause is postulated to be either inflammatory neovascularization or minimal thymic hyperplasia induced by Behçet's disease.