A theoretical model of heat balance is presented that could clarify the matching of babies' wrapping with their environments. Best estimates of metabolic heat input and heat loss by all known routes are defined for 22 parts of the body surface. The variation of these with core temperature, posture, skin vasodilatation, and the onset of sweating are calculated: first, by using presumed skin temperatures and second, by following iterative calculation of the skin temperature and the consequent total heat losses. Calculation of the highest tolerable ambient temperature (HTAT) for a given set of clothes, underbedding, and covers shows that a well-wrapped baby lying face down could have an HTAT 10 degrees C lower than if he/she were lying supine. Representative values for highest and lowest tolerable temperatures (defined in text) are presented for the first 6 mo of life. Retrospective estimation of thermal balance from death-scene data on clothing and bedding can permit assessment of hyperthermia or hypothermia as a contributing cause of death. Recommendations are made on the avoidance of hyperthermia.