Environmental heat is a health risk especially for the elderly and those with a pre-existing disease. Mortality effects of prolonged heat waves in Finland were studied for different age groups, genders, causes of death, and places of death. Mortality due to various causes of death increased significantly in people aged 75 or above. The increase in risk was higher in health care facilities than other places of death. In the future, more efficient mitigation of adverse health effects of heat is needed as the population ages and climate change proceeds. Special attention should be given to the conditions in health care facilities.