During the 2003 heat wave an increase in mortality was observed in several European countries. Evidence suggests that the heat wave effect on mortality varies based upon underlying disease. In this study we examined the effects of the 2003 heat wave on all-cause and cause-specific mortality (neoplasms, cardiovascular and respiratory diseases) in a large west German city. Daily weather data for Essen was obtained from the German meteorological service. Death certificates for all deaths in Essen from 2002 to 2003 were coded according to the World Health Organization (WHO) guidelines. Mean numbers of daily deaths during and after the heat wave were compared with the average mortality in summer months (reference period). Poisson generalized additive models, adjusted for weekday and season, were fitted for overall and cause-specific mortality for the entire study period. During the 2003 heat wave (August 6-12), daily mortality increased by 15% (neoplasms), 30% (cardiovascular), and 61% (respiratory), with a decrease in the week after the heat wave of 17% for neoplasms and a sustained rise for respiratory mortality (77%). Regression analysis showed an association between heat and overall mortality in 2003 and greatest associations for respiratory mortality. Even the comparatively short heat wave in Essen in the year 2003 was associated with a rise in overall and cause-specific mortality. Different mechanisms appear to influence cause-specific mortality, with strongest associations for respiratory mortality. Harvesting might play a role in mortality due to neoplasms.