During the summer of 2003, Europe experienced a heat wave that lasted almost 2 weeks in which high temperatures were accompanied by exceptionally high ozone levels. Unfortunately, few studies have examined the effects of temperature and ozone simultaneously. The authors use constrained distributed lag models to estimate the effects of daily temperature and peak ozone on mortality in 12 French cities during the heat wave and to estimate the deaths attributable to each component. Elevated minimum and maximum temperature and peak ozone all increase mortality, with substantial interaction effects between temperature and ozone. Researchers expect extreme weather events, along with elevated ozone levels and temperatures, to become more common. Our results suggest that ozone will be an important contributor to the adverse health effects of such events.