There is great interest in the study of brown fat and its importance in human metabolism. Since the routine use of fluorodeoxyglucose (FDG) F-18 positron emission tomography-computed tomography (PET/CT) imaging, it has been recognized that brown adipose tissue (BAT) is often visualized in adults. BAT typically appears as symmetrical uptake of FDG, most frequently seen in the cervical/supraclavicular region. In this report, we describe 2 cases with Horner syndrome and 1 case with surgical sympathectomy, which had unilateral FDG uptake in BAT contralateral to the affected side. This imaging study emphasizes the importance of sympathetic innervation in activation of BAT. Adipose tissue is classified by its coloration in 2 types, white adipose tissue and BAT. White adipose tissue is composed of a single lipid droplet and acts as the primary site of energy storage and as a modulator of whole-body metabolism and insulin resistance. BAT is characterized by small cells, multilocularity, large number of mitochondria, increased vascularity, and abundant sympathetic noradrenergic innervations. The brown adipocyte itself has an uncoupling protein 1 that mediates thermogenesis. Thermogenesis in BAT is controlled by norepinephrine released from the sympathetic nervous system; which interacts mainly with beta-adrenergic receptors in BAT. BAT uptake of FDG, a surrogate for imaging glucose metabolism, and of I-123 metaiodobenzylguanidine (MIBG) and F-18 fluorodopamine, surrogates for imaging sympathetic innervation, has been described. A recent study with FDG showed that functional BAT is prevalent in adult humans, with a significant female predominance. The most conspicuous BAT depot detected in the human adult with PET-CT technique localizes to the supraclavicular/cervical area; but it may also be found in paravertebral, mediastinal, and perinephric fat. Physiologic uptake in the supraclavicular and cervical region is generally symmetric. The use of PET imaging may provide insight into physiology and regulation of BAT. We present 3 cases that illustrate, with imaging, the importance of sympathetic innervation in activation of BAT.