Reports the findings of a study of the prevalence of and recent trends in asthma among adults residing in metropolitan and non-metropolitan counties in the United States. In 2003, the adjusted prevalence of lifetime asthma diagnosis was 12.0 percent for metropolitan counties and 11.0 percent for non-metropolitan counties (p < 0.001). Prevalence of lifetime asthma diagnosis trended upwards across the rural-urban spectrum between 2000 and 2003, and states with the highest 2003 prevalence and the greatest increase in prevalence among non-metropolitan residents were concentrated in the West Census region (e.g., Arizona, California, Oregon and Washington). Asthma prevalence in non-metropolitan counties was highest for those aged 18 to 34 (15.9%), the unemployed (13.5%), American Indians (12.7%) and women (12.4%).