Uptake of (123)I-MIBG in the neck and shoulders has recently been shown to be caused by uptake in brown adipose tissue. Unilateral absence of (123)I-MIBG uptake in brown adipose tissue ipsilateral to the clinical findings of a post-surgical Horner's syndrome suggests that, in humans, as in the animal model, uptake in brown adipose tissue is mediated by the sympathetic nervous system. This case further elucidates the mechanism of radiopharmaceutical uptake in brown adipose tissue and also suggests possible models for future studies of the physiology and pharmacology of brown adipose tissue.