Rationale: While the association between mortality and particles is well established, fewer studies have been reported with ozone. The harvesting hypothesis posits that the deaths associated with an exposure are occurring in people who are dying already, and the effect of exposure is merely to move the death from one day to an earlier day, and has no other effects that would influence deaths. Objectives: The aim of this study was to analyze the effect of ozone on mortality, and the extent to which this is due to short term mortality displacement in 48 US cities between 1989 and 2000. Methods: Time series of mortality and ozone were investigated with a generalized linear model during the June-August months, controlling for season, day of the week and apparent temperature. We examined an unconstrained and a smooth distributed lag with 21 lags of ozone, and effect modification for city-specific characteristics. Main Results: We found a 0.3% (95%C.I.: 0.2-0.4) increase in total mortality for 10 ppb increase in 8-hr ozone at lag 0 during summer months. The effect increased to 0.5% (95%C.I.: 0.05-0.96) when looking at the unconstrained distributed lag. The shape of the distributed lag indicates that all the effect is in the first week. Conclusions: We didn't find mortality displacement due to ozone, rather the effect size estimate when looking at 21 days of ozone was larger than when using a single day's ozone concentration. Hence these results indicate that risk assessments using the single day of ozone exposure are likely to underestimate, rather than overestimate, the public health impact.