Brown adipose tissue (BAT) densities assessed as CT Hounsfield units (HUs) were evaluated in a rodent model and in patients to determine whether HUs changed in relation to BAT activity. Serial ¹⁸F-FDG PET/CT was performed on rats under both room temperature control conditions and after 4 h of cold-stimulation, which is known to activate BAT. The maximum standardized uptake values and CT HUs of BAT were measured, and tissues were examined in the laboratory. Image records from cancer patients who underwent PET/CT were reviewed, and 23 patients were identified who displayed both high and low ¹⁸F-FDG uptake into BAT on serial ¹⁸F-FDG PET/CT scans. The maximum standardized uptake values and CT HUs of BAT were compared in these scans. The mean (±SD) CT HUs of cold-activated BAT (−12.4 ± 22.4) were significantly higher than those (−27.9 ± 9.6) of the controls in the rat model. The CT HUs of BAT (−71.6 ± 18.0) in the patients with high ¹⁸F-FDG uptake were significantly higher than those (−104.4 ± 16.8) of the patients with low ¹⁸F-FDG uptake . A decrease in relative lipid content is seen in activated BAT in rats on histology. The CT HUs of BAT increased in activated conditions in both animals and patients, likely because of lipid consumption by activated BAT.