The summer of 2003 was exceptionally hot, leading to an excess of mortality in Europe. Here, we assess the short-term effects of extreme hot summer temperatures on total daily mortality in Barcelona (Spain). Daily mortality from burial records, maximum temperature, relative humidity and photochemical pollutants, were collected for the period 1999-2003. Data was analysed using Poisson regression with generalised additive models. Mortality shows a considerable increase when maximum temperatures are over a threshold temperature of 30.5 degrees C. The risk of death associated with an increase of 1 degrees C above the threshold was 6%, 7% and 5% after 1, 2 and 3 days, respectively. Exposure to extreme hot temperatures leads to an significant increase in mortality.