Observations and experiments in animals and human beings grant plausibility to the hypothesis that hypothermia is a risk factor for pneumonia. Exposure of body to cold stress causes alterations in the systemic and local defenses against respiratory infections, favoring the infection by inhalation of pathogens normally present in the oropharynx. Neonates and young infants with hypothermia have an increased risk of death; however, there is no strong demonstration that hypothermia leads to pneumonia in these children. Studies that properly addressed the problem of confounding variables have shown an association between cold weather and pneumonia incidence. Probably the strongest evidence that supports the plausibility of the hypothesis is provided by the controlled comparison between patients with traumatic brain injury treated with hypothermia and those treated under normal body temperature. The association between exposure to cold and pneumonia is strong enough to warrant further research focused in young children in developing countries.