A large number of studies have established the mitochondrial uncoupling protein UCP1 as a specific marker of brown adipocytes, where it controls energy dissipation of fatty acid oxidation as heat in response to physiological requirements. Following the recent report of the detection of UCP1 in thymocytes of rats and mice, we reinvestigated its presence in thymus. Light microscopy and immunohistochemical analysis demonstrated that the UCP1 signal in thymus is entirely explained by the presence of typical brown adipocytes around the gland. Staining for UCP1 was not observed in thymocytes. Similarly, UCP1 failed to be observed in rat spleen, skeletal muscle, stomach, intestine, or uterus, even after exposure of animals to the cold. These data confirm the specificity of UCP1 expression in the thermogenic brown adipocytes and argue against a direct role for this mitochondrial transporter in immune cells. Whether brown adipocytes adjacent to thymic lobes play a role in thymus physiology remains to be investigated.