Objective: To study the relationship between mortality and temperature in Cantabria, a Spanish region that includes both rural and urban areas. Methods: Meteorological data (2003-2006) were obtained from the Spanish Meteorological Agency and daily numbers of deaths were obtained from the Spanish Institute for Statistics. A graphical approach using locally weighted regression smoothing was used to explore the relationship between mortality and temperatures and to identify temperature thresholds; we estimated the excess of mortality due to extreme temperatures in both warm and cold periods using Poisson regression models, and we simulated a situation with increased temperatures. Results: Raising maximum or minimum temperatures by 1oC was associated with a 2% excess in mortality risk in the whole population throughout the warm period, and we found no effect in mortality on the cold season; almost all changes in mortality occur in people aged 65 or more. Women are more sensitive to temperature changes in the warmer months. Conclusions: The deleterious effect of increasing temperatures in summer is more pronounced than the beneficial effect of a similar increase in winter. © 2011 Swiss School of Public Health.