West, R. R. (Welsh National School of Medicine, Dept. Community Medicine, Heath Park, Cardiff CF4 4XN, Wales) and Lowe, C. R. Mortality from ischaemic heart disease intertown variation and its association with climate in England and Wales. International Journal of Epidemiology 1976, 5: 195—201. In both seasonal and inter-town variation, the mortality from ischaemic heart disease in England and Wales in 1969–71 is very highly correlated with temperature. Also in the inter-town variation, IHD mortality is highly correlated with rainfall and with socio-economic index. It is postulated that the same mechanism of body cooling underlies seasonal, intertown and socio-economic variations in IHD mortality. The first law of thermodynamics states that heat and energy are equivalent. Therefore to maintain body temperature against an increased thermal gradient (caused by a lower environmental temperature as in seasonal variation, or a temperature-rainfall interaction as in inter-town variation) more energy must be expended by the body or, alternatively, the body must reduce heat loss by increasing thermal insulation. It is to be expected that there will be many accompanying changes in physiological parameters and several of these are discussed.