Focal mediastinal F-18 FDG uptake may be from potential adenopathy requiring biopsy confirmation or benign active brown adipose tissue to be left untouched. Knowledge of this potential pitfall and precise localization with fusion PET/CT are important in preventing misinterpretation as malignancy. Our case report is important in the aspect that CT was not able to confirm the uptake as benign finding, which led to invasive biopsy and biopsy confirmed it to be a benign brown adipose tissue.