The aim of this study was to assess the volume and function of human brown adipose tissue (BAT) in vivo using MR imaging. 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 ¹⁸F-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. 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 ¹⁸F-FDG PET/CT scans. During mild cold challenge, BOLD fMRI signal increased in BAT by 10.7% ± 1.8% (P < 0.01). We demonstrated the feasibility of using MR imaging and fMRI to assess BAT volume and BAT responses to mild cold stimulation in the cervical areas of human subjects.