Intra-abdominal adipose tissue was obtained at laparotomy from three subjects with high circulating noradrenaline concentrations in the presence of phaeochromocytoma. Light and electron microscopy confirmed typical brown adipose tissue, both adjacent to, and in one case distant from, the tumour. Biochemically, in terms of high cytochrome-C oxidase activity, mitochondrial GDP-binding, GDP-inhibitable uncoupled mitochondrial respiration, and specific concentration of uncoupling protein (mean 31 +/- 7 micrograms/mg mitochondrial protein) the tissue possessed all the unique features of thermogenically active brown adipose tissue. These findings are contrasted with low results obtained from a case with Cushing's disease, and the significantly lower results (mean 2.5 +/- 1.8 micrograms/mg) in a group of control adults (P less than 0.02). In the presence of high circulating noradrenaline concentrations, the intra-abdominal fat of human adults, including the omental fat, which is brown adipose tissue in infancy, becomes reactivated and may be contributing to the weight loss which is typically seen with phaeochromocytoma. Human adult brown adipose tissue thus has the biochemical potential for the thermogenic activity required in order to contribute to the regulation of energy balance and body weight.