UNLABELLED: The aim of this study was to assess the volume and function of human brown adipose tissue (BAT) in vivo using MR imaging. METHODS: BAT volumes under thermoneutral conditions in the cervical areas were assessed via water-fat contrast using the Dixon method and via water-saturation efficiency using fast spin-echo and T2-weighted images. The existence of cervical BAT was also assessed by (18)F-FDG PET/CT scans in the same subjects. BAT functionality was assessed via functional MR imaging (fMRI) blood oxygenation level-dependent (BOLD) signal changes in response to a mild cold challenge. RESULTS: Under thermoneutral conditions, we were able to distinguish BAT from white adipose tissue in the cervical and supraclavicular fat. BAT showed higher water-to-fat contrast and higher water-saturation efficiency in MR imaging scans. The location and volume of BAT assessed by MR imaging were comparable to the measurements by (18)F-FDG PET/CT scans. During mild cold challenge, BOLD fMRI signal increased in BAT by 10.7% ± 1.8% (P