During airborne field studies in 1999 and 2000, we studied the photochemical production of ozone in four U.S. urban areas: Nashville, TN; Atlanta, GA; Houston TX and Dallas-Ft. Worth TX. These areas differed widely in population, magnitude of ozone precursor sources, relative contributions to VOC emissions (vehicular, petroleum refining, biogenic), point source contribution to NOx emissions, and boundary layer meteorology (coastal versus inland). The ambient ozone levels were uniquely high in Houston TX area (up to 250 ppbv) compared to the other three areas (< 140 ppbv). We will compare and contrast the precursor concentration distributions and the photochemical processing of the NOx precursors in these four urban areas. We conclude that the combination of boundary layer meteorology that confines plumes plus intense emissions of fossil fuel combustion mixed with very reactive hydrocarbons emitted by petrochemical industries are necessary and sufficient for formation of the high ozone levels unique to the Houston TX area.