Brown adipose tissue (BAT) provides non-shivering thermogenesis. In humans, active BAT can be visualized by its 18F-fluoro-desoxyglucose (FDG) uptake as detected by positron emission tomography (PET) combined with computer tomography (CT). The retrospective analysis of clinical scans is a valuable source to identify anthropometric parameters that influence BAT mass and activity and thus the potential efficacy of envisioned drugs targeting this tissue to treat metabolic disease. We analyzed 2854 FDG-PET/CT scans from 1644 patients. We identified 98 scans from 81 patients with active BAT (BAT+). We quantified the volume of active BAT depots (mean values in ml ±SD (n): total BAT 162±183 (98), cervical 40±37 (53), supraclavicular 66±68 (71), paravertebral 51±53 (69), mediastinal 43±40 (51), subphrenic 21±21 (29)). Since only active BAT is detectable by FDG uptake, these numbers underestimate the total amount of BAT. Considering only 32 scans of the highest activity as categorized by a visual scoring strategy, we determined a mean total BAT volume of 308±208ml. In 30 BAT+ patients with three or more repeated scans we calculated a much higher mean probability to re-detect active BAT (52±25%) as compared to the overall prevalence of 4.9%. We analyzed parameters that characterize BAT+ subjects by comparing them to the overall cohort and to an age- and sex-matched control group. We observed a higher proportion of young (p<0.001) and female (p<0.001) patients in comparison to the total cohort and, in the case-control design, a clear annual pattern of less BAT+ scans during summer and an according difference in mean outdoor temperature during scans (p=0.03). We also analyzed parameters that influence the extent of activation within the BAT+ group by calculating a BAT activity index (BFI) based on volume and intensity of individual BAT depots. By a linear regression model we detected higher total BFI in younger patients (p=0.009), while sex, BMI, height, mass, outdoor temperature and blood parameters did not affect total or depot specific BAT activity. Surprisingly, renal clearance as estimated from mass, age and creatinine was a significant predictor of BFI on the total (p=0.005) as well as on the level of several individual depots. In summary, we detected an amount of active BAT higher than previously reported. BAT-positive patients represent a group with higher than usual probability to activate BAT during a scan. This characteristic was found more often in young, female subjects and was amplified by cold weather. Estimated renal clearance correlated with the extent of activated BAT in a given scan, probably as a secondary reporter of sympathetic tone. These data imply an efficacy of drugs targeting BAT to treat metabolic disease that is at the same time higher and subject to a larger individual variation than previously assumed.