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Energy balance studies were done during 10–29 days on 15 immature infants of mean birth weight 1581 g. Mean gross energy intake was 757 kJ/kg (181 kcal) and 79 % of this was retained, so that metabolizable energy was 602 kJ/kg (143 kcal). Mean resting metabolic rate was 244 kJ/kg (58.1 kcal), and it increased with advancing maturity. Minimum resting metabolism averaged 199 kJ/kg (47.5 kcal). Energy expended in activity increased with maturity, but amounted to less than 17 % of the total energy turnover. Postprandial metabolism caused the mean V̇O2 to rise by 17 % in the hour after a feed, and during 24 hr resulted in consumption of energy equivalent to about 10 % of the resting metabolism. Stored energy amounted to 230 kJ/kg (55 kcal) and was linearly related to weight gain (r = 0.92). Energy cost of weight gain was 24 kJ/g (5.7 kcal) and energy stored in new tissue was 16.8 kJ/g (4.0 kcal). Maintenance energy requirement at zero growth rate was about 270 kJ/kg (64 kcal).Data on energy balance in immature infants are scanty. There is need for information on energy retention, maintenance energy requirements, postprandial energy expenditure, and energy cost of growth. We have attempted to provide such information, but our data show wide variation between infants. However, in spite of this variation, a clear relationship is present between energy retention and weight gain, the main limiting factor in weight gain probably being energy absorption from the gut. We need more data to show whether efficiency of energy utilization alters with different levels of intake and at different ages.