PURPOSE: To evaluate the volume and changes of human brown adipose tissue (BAT) in vivo following exposure to cold using magnetic resonance imaging (MRI). MATERIALS AND METHODS: The clavicular region of 10 healthy volunteers was examined with a 3T MRI system. One volunteer participated twice. A cooling vest that was circulated with temperature-controlled water was used to expose each volunteer to a cold environment. Three different water temperature phases were employed: baseline (23°C, 20 min), cooling (12°C, 90 min), and a final warming phase (37°C, 30 min). Temperatures of the water in the circuit, of the body, and at the back skin of the volunteers were monitored with fiberoptic temperature probes. Applying the 2-point DIXON pulse sequence every 5 minutes, fat fraction (FF) maps were determined and evaluated over time to distinguish between brown and white adipose tissue. RESULTS: Temperature measurements showed a decrease of 3.8 ± 1.0°C of the back skin temperature, while the body temperature stayed constant at 37.2 ± 0.9°C. Focusing on the two interscapular BAT depots, a mean FF decrease of -2.9 ± 2.0%/h (P < 0.001) was detected during cold stimulation in a mean absolute volume of 1.31 ± 1.43 ml. Also, a correlation of FF decrease to back skin temperature decrease was observed in all volunteers (correlation coefficients: |r| = [0.51; 0.99]). CONCLUSION: We found that FF decreases in BAT begin immediately with mild cooling of the body and continue during long-time cooling.