Fluorine-18 fluoro deoxyglucose positron emission tomography (F-18 FDG PET) imaging is an exciting modality for the study of pathophysiology. The fusion of computed tomography (CT) anatomic data to metabolic F-18 FDG PET data allows accurate mapping of the metabolic activity of the body. With each patient evaluated with fusion PET/CT imaging, valuable information is obtained regarding anatomic and physiological abnormalities and normal variants. We describe 4 cases in which the unusual distribution of increased FDG metabolism localized to fat, which we propose represents brown adipose tissue. The accurate anatomic localization of this hypermetabolic fat is aided by imaging these patients on a combined PET/CT system.