Brown adipose tissue (BAT) plays a major role in energy homeostasis in animals. Detection of BAT using positron emission tomography (PET)-CT in humans has challenged the view that BAT disappears after infancy. Several recent studies, based on analysis of single scans, have reported a low prevalence of only 5–10% in humans, casting doubt on its significance. We undertook a critical analysis of the sensitivity, reproducibility, and accuracy of PET-CT to deduce the prevalence of BAT and factors associated with its detection in adult humans. In a retrospective evaluation of PET-CT, using [¹⁸F]fluorodeoxyglucose, performed in 2,934 patients, BAT was identified in 250 patients, yielding an apparent prevalence of 8.5%. Among those patients with BAT, 145 were scanned more than once. The frequency of another scan being positive increased from 8 to 65% for one to more than four additional studies. The average probability of obtaining another positive scan among patients with BAT is 13%, from which the prevalence of BAT is estimated at 64%. BAT was more commonly detected in women, in younger (36 ± 1 vs. 52 ± 1 years, P < 0.001) and leaner (20.1 ± 0.9 vs. 24.9 ± 0.9 kg/m², P < 0.01) individuals. Fasting glucose was lower in those with BAT than those without (4.9 ± 0.1 vs. 5.5 ± 0.1 mmol/l, P < 0.01). Among patients scanned more than once, BAT was detected when body weight and fasting glucose were lower (54.9 ± 0.5 vs. 58.2 ± 0.8 kg, P < 0.001 and 4.9 ± 0.3 vs. 5.5 ± 0.3 mmol/l, P = 0.03). We conclude that BAT is present in the majority of adult humans. Presence of BAT correlates negatively with body mass index and glucose concentration. BAT may play an important role in energy homeostasis in adults.