The ultrastructure of brown adipose tissue (BAT), the thermogenic type of adipose tissue, was investigated in biopsies from 4 pre-term human new-borns delivered at 25-27 week's gestational age and compared with peri-renal brown fat from 2 adult patients with phaeochromocytoma (a condition of brown fat activation). The cell size of brown adipocytes was smaller in pre-term new-borns than in adult patients; adipocytes were almost exclusively multilocular, suggesting active thermogenesis. In 3 of the pre-term new-borns, brown adipocyte ultrastructure indicated a good to high degree of differentiation (in particular at the level of mitochondria) as compared with activated brown fat cells found in adult patients; in one pre-term infant the tissue morphology was obviously suggestive of an earlier, proliferative phase of development and the differentiation process of brown adipocytes could be traced in some detail. The results suggest that (a) brown adipose tissue may be fairly well-differentiated and thermogenetically active in pre-term human new-borns weighing about 750 g at birth; (b) brown adipocytes apparently develop from vessel-associated cells, the early signs of adipocyte differentiation being glycogen and lipid accumulation; (c) the ultrastructural morphology of mitochondria of mitochondria in well-differentiated BAT from pre-term infants can strictly resemble that found in active brown adipose tissue of adult phaeochromocytoma patients.