The Municipality of Anchorage. Alaska has an emissions inspection and maintenance (I/M) program. The program was designed to reduce carbon monoxide (CO). During the first year of operation I/M repairs reduced the fleetwide idle CO emissions by 48.7% and the 2500 RPM CO emissions by 41%.Since the start up of the program there has been a reduction in ambient CO concentrations and a sharp reduction in the number of days the federal CO standard has been exceeded. Much of this may be due to milder winter temperatures observed since I/M began. To determine if the ambient reduction is due solely to weather, a multiple regression analysis was developed. The analysis shows that the ambient CO reduction achieved in Anchorage can not be attributed solely to weather conditions. A review of data concerning average vehicle miles traveled per day, population changes, and CO control strategies indicates that the main factors causing the non-weather related CO reductions are the federal new car CO standards and the I/M program.