The U.S. Environmental Protection Agencys current Speciation Trends Network (STN) covers most major U.S. metropolitan areas and a wide range of particulate matter (PM) constituents and gaseous co-pollutants. However, using filter-based methods, most PM constituents are not measured daily and the lack of daily air quality data complicates epidemiologic analyses of the potential adverse health effects of these PM constituents. Possible criteria for the identification of metropolitan areas with the greatest epidemiologic value for enhanced monitoring are population, mean levels and variation of criteria air pollutants and PM constituents, correlations among these pollutants, and the relationship of these correlations to the coefficient of variation. Using a review of air quality measurements from 49 STN monitors for 2001-2005 as an illustration of this criteria, we selected metropolitan areas that had the appropriate population size, sufficient PM(sub 2.5) concentration levels, variability for most pollutants, and appropriate correlations between pollutants.