In the United States, exhaust emission performance has improved dramatically since the precontrol days of the 1960's with emission reductions typically exceeding 90 percent. However, these emission reductions and emission compliance in general have traditionally been evaluated at warm ambient conditions with a nominal test temperature of 75°F. This paper examines the emission performance of current technology vehicles under colder ambient test conditions. Emission levels at 20°F are found to be three to four times the 75°F levels. The bulk of this emission increase occurs during the cold start portion of the test where increased fuel enrichment and decreased emission control system efficiency combine to raise the tailpipe emission levels. In comparison to precatalyst vehicle designs, emission reductions have not been as great at colder ambients as at the normal, warm test conditions. This lag in emission control exacerbates efforts to improve air quality at cold temperatures which is particularly a concern for carbon monoxide.