Scragg R [Commonwealth Scientific and Industrial Research Organization, Division of Human Nutrition, Kintore Avenue, Adelaide, South Australia 5000.] Seasonality of cardiovascular disease mortality and the possible protective effect of ultra-violet radiation. International Journal of Epidemiology 1981, 10: 337–341.Previous studies have suggested that cardiovascular disease (CVD) seasonality is due to variations in temperature or respiratory disease prevalence. Another mechanism might be that the seasonal variation in ultra-violet (UV) radiation is responsible for the seasonality of CVD. An hypothesis is put forward that UV radiation, by increasing body levels of vitamin D, protects against CVD by decreasing the risk of thrombus formation. This hypothesis might explain the seasonal variations in CVD mortality and morbidity which decrease in summer, the higher CVD mortality in higher latitudes, and the inverse relationship between altitude and CVD mortality. It is speculated that this mechanism might Involve a direct effect of vitamin D on the platelet, or might be mediated by a change in calcium matabolism.