In Finland, mortality increases steeply in autumn, reaches a peak during the Christhmas holidays and declines slowly towards a trough in August. The relative excess in daily mortality (peak vs. trough) is 30% for coronary heart disease, 40% for cerebral vascular accidents and 90% for diseases of the respiratory organs. There is a secondary peak in Midsummer, especially in coronary deaths of working aged men. Mortality is lowest at mean daily temperature of +14 degrees C, and it increases slowly with falling temperature and steeply with increasing temperature. An estimated 2000-3000 extra deaths occur in Finland during the cold season, most of which are people aged 65 years and over but 20% at working age. The number of people dying from high temperatures (over +14 degrees C) in this country in a normal year is 100-200. Heat deaths are mostly certified as being due to cardiovascular or respiratory conditions. Exposure to cold air causes a rise in blood pressure and haemoconcentration which lead to increased tendency to vascular thromboses. In hot weather, haemoconcentration due to sweating and perspiration increases blood viscosity and the risk of thrombosis. Both cold and heat are significant public health hazards which should be taken into account in health care and education of health professionals.