ABSTRACT. Objectives. To determine the role of heat stress in sudden infant death syndrome (SIDS) by examining the SIDS rates during periods of extreme environmental temperatures in a period when most infants were placed prone for sleep.Design. A retrospective study of SIDS rates and mortality rates attributable to excessive environmental heat in relationship to climatologic temperature was performed. Data were collected for each of 454 counties in 4 states (Arkansas, Georgia, Kansas, and Missouri) from May 1 to September 30, 1980, and were then summed to yield the mortality rates for each 5°F (2.8°C) temperature range.Results. 2 analyses revealed significant relationships for heat-related mortality rates and both the maximal daily temperature and mean daily temperature but demonstrated no such relationships for SIDS rates. No association between SIDS rates and heat-related mortality rates was found with Spearman's ranked correlation, for either the maximal daily temperature or the mean daily temperature.Conclusions. On the basis of our findings of no significant association between SIDS and either measure of temperature during periods of high heat stress-related death rates, it seems unlikely that the heat stress associated with the combination of prone sleep positions and elevated environmental temperatures has a significant role in the development of SIDS. Pediatrics 2004; 113:e586 -e592. URL: http://www.pediatrics.org/cgi/ content/full/113/6/e586; SIDS, thermal stress, bedding, overwrapping, sleep position.ABBREVIATIONS. SIDS, sudden infant death syndrome; T max , maximal daily temperature; T avg , average daily temperature. T he thermal stress theory for sudden infant death syndrome (SIDS) states that many SIDS deaths occur as a result of increased demands on thermoregulatory mechanisms attributable to a prone sleep position, increased ambient temperature, and/or a decreased ability to eliminate body heat as a result of excessive clothing covering the infant. 1 Although the relationship between climatic temperature and SIDS incidence has been examined carefully, the studies were performed predominantly in nations without extreme temperature variations, such as Great Britain, Tasmania, and New Zealand. [2][3][4][5] Such research has consistently associated increased SIDS incidence with colder outdoor temperatures. It has been suggested that some parents may respond to such cold waves by overwrapping infants or excessively increasing room temperatures. 3 In contrast, a cross-cultural study found comparatively weak relationships between climatic temperature and SIDS incidence in communities in the United States. 6 Therefore, it is possible that seasonal overwrapping of infants may not be a major cause of SIDS in the United States.Sections of the United States, particularly the Midwest, are subject to periodic heat waves, which can lead to severe epidemics of deaths attributable to exposure to excessive heat. 7 Such conditions present an opportunity to examine the effect of increased environmental temperature o...