High temperatures have a direct impact on body functions. Heat waves increase mortality risks due to myocardial infarction, stroke, and pulmonary disease. Cold temperatures also increase mortality, but with a longer latency. A recent study found only a small difference between the minimal mortality temperature (MMT) and the temperatures at which mortality rose steeply, although the majority of deaths occurred at temperatures below MMT. Global climate change with increasing temperatures seriously threatens health, work capacity, and generation of household incomes, particularly among poor people in hot countries. In Sweden, heat waves increase mortality in vulnerable groups of elderly people and patients with chronic heart and lung diseases, as well as those performing intensive physical work in hot environments. The medical profession can play an important role not only in prevention of climate change, but also in adaptation to climate change with the goal of minimizing health risks.