Ozone is the only pollutant that exceeds national and state standards in Delaware. Using observations and two different climate models, the number of high-ozone days (days exceeding 70 ppb based on the 8-hour average in Delaware) is investigated for the late 20th and early to mid-21st centuries using a synoptic typing methodology, which relates surface conditions conducive to high-ozone events to atmospheric circulation. High-ozone days are associated with the absence of precipitation and southwesterly to west-northwesterly flow over Delaware, which tend to bring higher daily mean temperatures (exceeding 25.5°C). Models underestimate the number of observed high-ozone days in the 20th century, because the models do not include the effects of ozone regulation, which has decreased the number of ozone days. Meanwhile, higher concentrations of greenhouse gases and the resulting higher temperatures favor increased ozone days, an effect that is captured by the models. As temperatures continue to rise in the 21st century, climate projections indicate that high-ozone conditions will occur more frequently. By mid-century, the number of high-ozone days is projected to increase by about an extra day every two years, which is faster than it was in the previous 30 years. Thus global warming cancels out a quarter of the progress made in improving air quality in the state of Delaware, meaning that the air quality in mid-century is expected to be the same as it was around 2006. In a warming world, air quality standards will need to stricter to maintain or reduce the number of high-ozone events in Delaware.