Previous studies 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 metabolism.