BACKGROUND: Studies in animals indicate that brown adipose tissue is important in the regulation of body weight, and it is possible that individual variation in adaptive thermogenesis can be attributed to variations in the amount or activity of brown adipose tissue. Until recently, the presence of brown adipose tissue was thought to be relevant only in small mammals and infants, with negligible physiologic relevance in adult humans. We performed a systematic examination of the presence, distribution, and activity of brown adipose tissue in lean and obese men (in the Netherlands) during exposure to cold temperature. Brown-adipose-tissue activity was studied in relation to body composition and energy metabolism. METHODS: We studied 24 healthy men - 10 who were lean (body-mass index [BMI - the weight in kilograms divided by the square of the height in metres], <25) and 14 who were overweight or obese (BMI, ≥25) - under thermoneutral conditions (22°C) and during mild cold exposure (16°C). Putative brown-adipose-tissue activity was determined with the use of integrated ¹⁸F-fluorodeoxyglucose positron-emission tomography and computed tomography. Body composition and energy expenditure were measured with the use of dual-energy x-ray absorptiometry and indirect calorimetry. RESULTS: Brown-adipose-tissue activity was observed in 23 of the 24 subjects (96%) during cold exposure but not under thermoneutral conditions. The activity was significantly lower in the overweight or obese subjects than in the lean subjects (P=0.007). BMI and percentage of body fat both had significant negative correlations with brown adipose tissue, whereas resting metabolic rate had a significant positive correlation. CONCLUSIONS: The percentage of young men with brown adipose tissue is high, but its activity is reduced in men who are overweight or obese. Brown adipose tissue may be metabolically important in men, and the fact that it is reduced yet present in most over-weight or obese subjects may make it a target for the treatment of obesity.