Harmonic analysis is applied to daily temperature and Ischaemic Heart Disease (IHD) mortality for five UK counties in order to investigate the relationship between IHD mortality and temperature seasonality over the period 1974/75 to 1989/99. Specifically, relationships between the timing (phase) of the mortality peak and minimum temperatures and the seasonality (amplitude) of temperature and IHD mortality are explored. At the general level threshold temperatures appear to exist beyond which the scatter and severity of IHD mortality increases in all counties. Analyses of the relationships between temperature and IHD mortality phase and amplitude reveal statistically significant relationships between the strength of the seasonal cycle of temperature and IHD mortality and the timing of minimum temperature and the seasonal mortality peak. The intensity of the seasonal cycle of temperature and IHD mortality appears to reflect inter-annual variation in large-scale atmospheric circulation patterns. Although there is an association between the timing of the minimum temperature and the mortality peak this is limited in years with later mortality peaks possibly due to short-term habituation and reduced pools of susceptible individuals. Study results also reveal trends to warmer temperatures, the earlier occurrence of minimum winter temperatures, reduced seasonality of mortality and earlier mortality peaks. Some geographical trends in temperature and IHD seasonality relationships are found and attributed to acclimatisation but spatial patterns are complex.