Ozone acts as a greenhouse gas and controls much of the oxidizing capacity of the atmosphere. Photochemical production of ozone in urban areas (smog) is a serious environmental problem, but how far this process extends on regional or global scales remains a major unanswered question in atmospheric science. In summer, Bermuda basks in pristine marine air, but in the spring, episodes of high ozone are common. From meteorological analyses and observations of ozone, carbon monoxide, and reactive nitrogen compounds, we conclude that half or more of the excess ozone in Bermuda originates from air pollution over eastern North America. ¿ American Geophysical Union 1995