Background: Individuals of south Asian origin have a very high risk of developing type 2 diabetes compared with white Caucasians. We aimed to assess volume and activity of brown adipose tissue (BAT), which is thought to have a role in energy metabolism by combusting fatty acids and glucose to produce heat and might contribute to the difference in incidence of type 2 diabetes between ethnic groups. Methods: We enrolled Dutch nationals with south Asian ancestry and matched Caucasian participants at The Rijnland Hospital (Leiderdorp, Netherlands). Eligible participants were healthy lean men aged 18-28 years, and we matched groups for BMI. We measured BAT volume and activity with cold-induced 18F-fluorodeoxyglucose (18F-FDG) PET CT scans, and assessed resting energy expenditure, non-shivering thermogenesis, and serum parameters. This study is registered with the Netherlands Trial Register, number 2473. Findings: Between March 1, 2013, and June 1, 2013, we enrolled 12 participants in each group; one Caucasian participant developed hyperventilation after 18F-FDG administration, and was excluded from all cold-induced and BAT measurements. Compared with Caucasian participants, south Asian participants did not differ in age (mean 23·6 years [SD 2·8] for south Asians vs 24·6 years [2·8] for Caucasians) or BMI (21·5 kg/m2 [2·0] vs 22·0 kg/m2 [1·6]), but were shorter (1·74 m [0·06] vs 1·85 m [0·04]) and lighter (65·0 kg [8·5] vs 75·1 kg [7·2]). Thermoneutral resting energy expenditure was 1297 kcal per day (SD 123) in south Asian participants compared with 1689 kcal per day (193) in white Caucasian participants (difference -32%, p=0·0008). On cold exposure, shiver temperature of south Asians was 2·0°C higher than Caucasians (p=0·0067) and non-shivering thermogenesis was increased by 20% in white Caucasians (p<0·0001) but was not increased in south Asians. Although the maximum and mean standardised uptake values of 18F-FDG in BAT did not differ between groups, total BAT volume was lower in south Asians (188 mL [SD 81]) than it was in Caucasians (287 mL [169]; difference -34%, p=0·04). Overall, BAT volume correlated positively with basal resting energy expenditure in all assessable individuals (β=0·44, p=0·04). Interpretation: Lower resting energy expenditure, non-shivering thermogenesis, and BAT volumes in south Asian populations might underlie their high susceptibility to metabolic disturbances, such as obesity and type 2 diabetes. Development of strategies to increase BAT volume and activity might help prevent and treat such disorders, particularly in south Asian individuals. © 2014 Elsevier Ltd. Chemicals/CAS: fluorodeoxyglucose f 18, 63503-12-8; glucose, 50-99-7, 84778-64-3