OBJECTIVE Brown adipose tissue (BAT) is a potential source of false-positive findings on [(18)F] FDG PET. In this report, we have discussed the (18)F-FDG uptake mechanisms in BAT and have aimed to determine if dual time point PET imaging helps to differentiate BAT from malignant lesions. METHODS Patients with dual-time-point PET/CT scans were reviewed retrospectively and 31 cases (11 males, 20 females, age: 28.6±9.7) having hypermetabolic BAT were included for this study. (18)F-FDG uptake in BAT was quantitatively analyzed by maximum standardized uptake values (SUVmax), and average percent change in SUVmax of BAT between early and delayed images was calculated. RESULTS Compared to the initial scans, (18)F-FDG uptakes in BAT in delayed images were higher in 26 of the patients, and lower in one patient. In terms of body regions, (18)F-FDG uptake increased in 80.6%, remained unchanged in 5.5% and decreased in 13.9% of the body regions. Mean percent change in SUVmax, including all BAT regions, was 19.8±19.1% while the mean percent increase was calculated as 69±25% in regions where progressive accumulation was observed. The increase in SUVmax correlated with the time interval between the two scans. CONCLUSIONS Physiologic (18)F-FDG uptake in BAT increases over time and may mimic the behavior of malignant lesions on dual time point PET imaging. Without the exact anatomic definition of the CT scan, false positive interpretation of PET data may be possible in cases with atypical BAT.