Southern New Zealnd has one of the highest postneonatal mortality rates in the developed world (8.1/1000 livebirths) and 77% of these deaths are attributed to the sudden infant death syndrome (SIDS). Both hyperthermia and sleeping position have previously been implicated in SIDS. A theoretical model to estimate the thermal balance of infants used here shows that the head, and particularly the face, becomes the main route for heat loss when thick clothing and bedding are used. This thermoregulatory role could be compromised by the prone sleeping position. It is postulated that particular cultural combinations of infant care practices (sleeping position, clothing, bedding, and room heating) may facilitate hyperthermia and explain widely disparate rates of SIDS in different countries and ethnic groups.