Emission data were developed from a sample of resident vehicles in Leadville, Colorado at a nominal elevation of 10,000 feet above sea-level. Emissions concentration sampling was performed on each vehicle in the sample at loaded and unloaded engine operating conditions. Concentration data were then converted to mass data using equations developed from key-mode and mass emissions testing of vehicles operating in the Denver area. Mass emissions data were then combined with mass data developed from vehicles operating in a metropolitan area situated near sea-level (Chicago) to define the emissions versus altitude relationships for exhaust HC, CO and NOx. HC and CO emissions at idle were also examined with respect to the applicability of a statewide minimum HC/CO idle emission standard for the state of Colorado.