Long-term trends of median and 90th percentile summer afternoon O₃ concentrations were examined at 549 sites across the United States for the 1980–1995 period. Daily temperature data were used to account for the variability in O₃ concentrations associated with temperature. Both before and after segregating the O₃ data by temperature, trends were insignificant over most of the continental United States. No region of the United States experienced a significant increase in O₃ concentrations during the 1980–1995 period. Decreasing trends were predominantly clustered in the three largest metropolitan areas: New York City, Los Angeles, and Chicago. In these areas, additional sites with trends were identified in the temperature-segregated analysis. Correlation of trends with local anthropogenic emissions of nitrogen oxides (NO _x = NO + NO₂) and volatile organic compounds (VOC) indicates a greater frequency of decreasing trends for urban sites with high emission. National emission inventories for the United States indicate that anthropogenic VOC emissions decreased by 12% over the 1980–1995 period while NO _x emissions remained constant. The observed O₃ trends are consistent with the view that summertime O₃ production over the United States is NO _x -limited except in the largest metropolitan areas where it is partly VOC-limited.