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 (NOx=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 NOx emissions remained constant. The observed O3 trends are consistent with the view that summertime O3 production over the United States is NOx-limited except in the largest metropolitan areas where it is partly VOC-limited. ¿ 1998 American Geophysical Union