GLOBAL increases in concentrations of tropospheric ozone are of worldwide concern because of its potential to affect both human and ecological health(1). Ozone has been implicated in the declining health of some forest tree species(2,3) because of its effects on many growth-related processes in controlled studies(3-5). Despite strong evidence that growth of forest tree seedlings can be reduced by ambient levels of ozone(6), there has been little basis for evaluating the threshold and magnitude of direct effects on growth of mature, economically important forest trees. We describe here a five-year study of serial changes in stem circumference of 28 mature loblolly pine (Pinus taeda L.) trees. This study has defined a rough ozone response threshold and quantified short- and longer-term components of growth responses to varying ozone and climate variables. Ozone exposures at greater than or equal to 40 nl l(-1) interacted with low soil moisture and high air temperatures to reduce short-term rates of stem expansion. Annual growth was also inversely related to seasonal ozone exposure and soil moisture stress. Effects varied widely between individual trees and years. Future ozone effects on forests are likely to be influenced by climate change and by projected increases in regional ozone pollution in industrialized countries.