Detection and quantification of brown adipose tissue (BAT) mass remains a major challenge, as current tomographic imaging techniques are either nonspecific or lack the necessary resolution to quantify BAT mass, especially in obese phenotypes, in which this tissue may be present but inactive. Here, we report quantification of BAT mass by xenon-enhanced computed tomography. We show that, during stimulation of BAT thermogenesis, the lipophilic gas xenon preferentially accumulates in BAT, leading to a radiodensity enhancement comparable to that seen in the lungs. This enhancement is mediated by a selective reduction in BAT vascular resistance, which greatly increases vascular perfusion of BAT. This enhancement enables precise identification and quantification of BAT mass not only in lean, but also in obese, mouse phenotypes, in which this tissue is invisible to conventional tomographic imaging techniques. The method is developed and validated in rodents and then applied in macaques to assess its feasibility in larger species.