Small preterm infants usually require a heated environment to survive. After weaning, some infants become hypothermic, and eventually require external thermal support for an additional period. We hypothesized that preterm infants respond to weaning from an incubator by increasing their resting metabolic rate. Thermally stable infants were studied when they had reached a weight of at least 1600 g. Resting energy expenditure was measured 2 hours before weaning and at 6, 23, 30, and 53 hours after weaning. Two-way analysis of variance for repeated measures was used for analysis. Sixteen infants with mean birthweight of 1270 ± 375 g and gestational age 31 ± 2.3 weeks were studied. After weaning, there was a significant increase in energy expenditure from 95.0 ± 21.9 kcal/d in the incubator, to a 30-hours peak of 111.9 ± 10.5 kcal/d after weaning. Weaning from a convective incubator results in an increase in metabolic rate in very low birth weight infants. We speculate that the infants' ability to increase metabolic rate might influence weaning success.