To compare carbon monoxide (CO) concentrations in areas that used oxygenated fuels with CO concentrations in areas that did not, we analyzed ambient CO concentrations from 62 monitors in 11 western U.S. states from 1986 through 1992. Five metropolitan areas in three states used oxygenated fuels for at least two winter seasons during this period. We determined the decrement in CO concentrations for each monitor, comparing the winters of 1989-1991 to the winters of 1986-1988. Areas that used oxygenated fuels had a slightly greater decrease in CO concentrations than areas that did not (1.2 parts per million [ppm] vs. 0.6 ppm decrease [20.5% vs. 10.3%] in mean daily concentration, 2.8 ppm vs. 1.4 ppm decrease [23.8% vs. 11.3%] in maximum daily concentration, and 1.8 ppm vs. 0.7 ppm decrease [21.4% vs. 8.9%] in 8-hour maximum daily concentration).