Background <p>Brown adipose tissues (BAT) are highly vascularized, mitochondria-rich tissues involved in thermogenesis. Physiological [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) uptake in BAT can be influenced by several factors, including the use of medications that act on β-adrenergic receptors. Recently, increased [¹⁸F]FDG BAT uptake has been observed in elderly patients (≥ 60 years old) receiving β3-adrenergic receptor agonists. With the increasing use of β3-adrenergic receptor agonists for managing overactive bladder, there is limited understanding of their potential association with increased [¹⁸F]FDG BAT uptake. This study retrospectively investigated whether treatment with β3-adrenergic receptor agonists is associated with increased [¹⁸F]FDG BAT uptake among elderly patients. [¹⁸F]FDG positron emission tomography (PET) images were analyzed visually and through SUVmax measurements across eight predefined regions of interest (ROI): cervical, periclavicular, axillary, mediastinal, paravertebral, paraabdominal aortic, perirenal, and perisplenic regions. Patients’ medication history and clinical records were reviewed to assess β3-adrenergic receptor agonist use relative to their [¹⁸F]FDG PET study.</p> Results <p>Forty-four elderly patients, each with a single [¹⁸F]FDG PET scan, were included in the analysis. Among the eight ROIs, the perirenal region showed a statistically significant increase in [¹⁸F]FDG BAT uptake in patients receiving β3-adrenergic receptor agonists compared with those not receiving, based on both visual analysis (<i>p</i> &lt; 0.001) and SUVmax measurements (<i>p</i> = 0.030). All patients receiving β3-adrenergic receptor agonists demonstrated increased [¹⁸F]FDG BAT uptake in the paravertebral region.</p> Conclusion <p>In patients aged ≥ 60 years, β3-adrenergic receptor agonist therapy appears to be associated with increased [¹⁸F]FDG BAT uptake, particularly in the perirenal area.</p>