The heat loss coefficients of a group of infants have been calculated and compared to see if there is a difference in the ability to lose heat between prone and non-prone sleeping infants. For a group of 43 infants aged 4-29 weeks, a simple mathematical model of exponential cooling in a body has been fitted to the fall in rectal temperature which occurs in infants at bedtime. One of the parameters yielded by the fitting process is the coefficient of thermal heat loss. After validation against the estimated heat loss from supine sleeping infants, the heat loss coefficient was compared at different sleep positions and gender. The mean heat loss coefficient, measured from the non-prone sleeping infants (0.269 W/degrees C, S.D. 0.197) agreed well with the value calculated for supine sleepers with the same tog levels (0.4 W/degrees C). Prone sleeping infants were found to have a considerably smaller heat loss coefficient which was approximately 60% of the value for non-prone sleeping infants (P = 0.000097). Female infants were found to have a heat loss coefficient that was approximately 70% of that of male infants but this gender difference was only significant (P = 0.025) for non-prone sleeping infants. These results suggest that infants sleeping in the prone position may be unable to lose heat as rapidly as those infants sleeping non-prone.