Study objective -Seasonality of coronary heart disease (CHD) was examined to determine whether fatal and non-fatal disease have the same annual rhythm. Design -Time series analysis was carried out on retrospective data over a 10 year period and analysed by age groups (<45 to >75 years) and gender. (under 45 years) admitted to hospital there was a dominant spring peak and an autumn trough. A bimodal pattern of spring and winter peaks was evident for hospital admissions in older male age groups: with increasing age the spring peak diminished and the winter peak increased. In contrast, female hospital admissions showed a dominant winter/summer pattern of seasonal variation. In male and female CHD deaths seasonal variation showed a dominant pattern of winter peaks and summer troughs, with the winter peak spreading into spring in the two youngest male age groups. CHD incidence in women showed a winter/summer rhythm, but in men the spring peak was dominant up to the age of 55. Conclusions -The male, age related spring peak in CHD hospital admissions suggests there is an androgenic risk factor for myocardial infarction operating through an unknown effector mechanism. As age advances and reproduction becomes less important, the well defined winter/summer pattern of seasonal variation in CHD is superimposed, and shows a close relationship with the environment, especially temperature, or the autumn and early winter fall in temperature. (J Epidemiol Community Health 1995;49:575-582) Studies of seasonal variation in coronary heart disease (CHD) are almost entirely based on data derived from national registers of deaths. Studies based on seasonal variation of CHD hospital admissions are few. Dunnigan et all found a bimodal pattern of seasonal variation with spring and winter peaks in a study of 47 281 admissions to all Scottish hospitals in 1962-66 in the diagnostic category ICD 420

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The composition of the seasonality of total death was ascertained. Vascular disease seasonality constitutes more than half. The remaining seasonality is influenced by respiratory disease. Surprisingly and of possible importance cancer mortality was not seasonal. Deaths from 'all other disease' and from 'injuries' is seasonal. Seasonality increases with age. In coronary and cerebrovascular disease death has a large seasonal fluctuation. On the other hand hospital admissions and survivors have a minor seasonal fluctuation--only cerebrovascular admissions reaching the chosen level of significance with a small seasonal amplitude. For vascular disease the ranking of seasonal fluctuation from greatest to least is--death outside hospital, total death, death inside hospital, admissions, survivors. It is death outside hospital presumably 'sudden' that imposes seasonality on coronary disease in general. For respiratory diseases not only death but hospital admissions and survivors have high amplitude seasonality with a much greater fluctuation than for death in vascular disease.