Over the first three months of life the infant's metabolic rate rises, which, together with the rise in ratio of mass to surface area, means that the net heat loss per unit surface area is 50% higher in a 3 month old infant than in a neonate. This, together with the thicker layer of subcutaneous fat and more effective peripheral vasomotor response to cold in a 3 month old infant, means that thermal balance is shifted in favour of heat conservation. The head is the site of 40% of heat production and of up to 85% of heat loss in an infant in bed: covers rising up over the head could therefore result in acute thermal imbalance with a rise in brain temperature not necessarily accompanied by a rise in body temperature. In animal studies relatively small changes in hypothalamic temperature have profound effects on the control of respiration. Alternatively, a rise in metabolic rate (from an acute infection, for example) could result in a significant change in thermal balance. There is anecdotal evidence that heat stress may be associated with sudden infant death or with severe hypoventilation. In the Avon studies infants with SIDS, particularly those over 70 days of age, were more heavily wrapped and were more likely to have had the heating on all night than control infants matched for age, date, and neighbourhood. There was no significant excess of viral infections in the infants with SIDS, but those who had virus infections were much more heavily wrapped than control infants with similar infections, suggesting that the combination of heavy wrapping and virus infection may be more important than either factor alone. There is, therefore, some physiological evidence that infants of 2 to 3 months of age may be more vulnerable to heat stress than younger infants, and limited evidence, from clinical studies, that this may occur and be associated with some sudden deaths. The precise contribution of thermal stress and the mechanism by which it could cause death remain unclear.