Brown adipose tissue or brown fat is a distinctive form of lipid-containing con-nective tissue found in almost all mammalian species. It differs from yellow or white fat in its anatomical distribution, histological appearance, and function. The active brown fat cell has a roundcentrally placed nucleus and a granular cyto-plasm containing multiple fat vacuoles. It is thus often referred to as multilocular, in contrast to white fat, which is unilocular. The cytoplasmic granularity depends on the large number of mitochondria in the cell. When inactive the brown fat cell becomes unilocular and indistinguishable from the white fat cell on light microscopy. Animal experiments have shown that brown fat is the main site of non-shivering heat production under conditions of cold stress (Smith & Roberts, 1964; Dawkins & Hull, 1965). Its blood supply equals that of resting skeletal muscle (Hausberger & Widelitz, 1963) and its metabolism is primarily directed towards the production of large amounts of heat.

Early work on the occurrence of brown fat in the human was done by Cramer (1920) and by Wegener (1951). Renewed interest in the distribution of brown fat followed the elucidation of its function and has focussed mainly on thesituation in the neonatal period (Aherne & Hull, 1966; Itoh& Kuroshima, 1967; Heim et al. 1968). There has been little work on the occurrence of brown fat in the later stages of life.