Ambient air concentrations of photochemical oxidants, monitored over a five-year period in Philadelphia, Denver, and five regions in California, were examined for evidence of trends in air quality. Statistically significant trends were identified in the cases of Denver and the San Francisco Bay area. The oxidant air quality in the San Francisco Bay area was found to be significantly related to the rates of emission of oxidant precursors. The relation is positive in the case of hydrocarbons, but negative for oxides of nitrogen. Geographical and meteorological factors are also important in determining the distribution of oxidant levels, and various indexes of oxidant air quality show different statistical behavior in regard to the detection of trends.