There is considerable regional variation in postneonatal mortality rates within New Zealand. The majority of postneonatal deaths are recorded as sudden infant death syndrome, and the regional rates of these deaths show a significant negative correlation with the mean daily temperature and mean 0900 hr vapour pressure (p less than 0.001). Postneonatal mortality rates, excluding sudden infant death syndrome mortality, show significant positive correlation with the regional percentage of the population of Maori descent (p less than 0.001). Further investigation of environmental temperature or factors closely related to it may shed light on the aetiology of the sudden infant death syndrome.