ISEE-0731 Background and Objective: The Greek capital, Athens, lies in a valley surrounded by four mountains and the sea and has a population of more than 4 million. The urban green area per inhabitant is small (2 m2), hence the peri-urban green areas on the surrounding mountains are of great importance. The climate of the area as well as the observed increase in temperature makes forest fires a real threat to the environment during the summer. We investigated the effects of forest fires on the total and cause-specific mortality on the population of Athens, during 1998–2004. Methods: We used Poisson regression to investigate the effect of forest fires on mortality. Three dummy variables for forest fires were included in the model: one indicating the days with a major forest fire (>30,001,000 m2 burnt), a second indicating relatively smaller forest fires (1,001,000–30,000,000 m2 burnt) and a third for small fires (10,000–1,000,000 m2 burnt). We also controlled for air pollution, temperature (plus heat-waves), wind speed and direction, week day and time trend. Results: Small fires do not appear to have an effect on mortality. Medium sized fires are consistently associated with an increase in mortality but this does not reach the nominal level of statistical significance in most cases. Large fires are associated with a 60% (95%CI: 46%–77%) increase in the daily total number of deaths, 68% (95%CI: 48%–92%) increase in the cardiovascular number of deaths and 102% (95%CI: 48%–175%) increase in the respiratory number of deaths, over and above the effect of temperatures, heat-waves, and ambient particles. Conclusion: Forest fires are not only a threat to human health as an environmental disaster. They have also an effect on mortality, not associated with accidental deaths, which appears to be proportional to the size of the fire.