Carbon monoxide (CO) is a colorless, odorless, tasteless toxic gas produced by incomplete combustion in fuel-burning devices such as motor vehicles, gas-powered furnaces, and portable generators. Persons with CO poisoning often overlook the symptoms (e.g., headache, nausea, dizziness, or confusion), and undetected exposure can be fatal. Unintentional CO exposure accounts for an estimated 15,000 emergency department visits and 500 unintentional deaths in the United States each year. The most recent state-level estimates of CO-related deaths were described in 1991 for the years 1979-1988. Using the most recent mortality data available, this report updates national and state-specific unintentional, non-fire-related CO mortality rates and describes the demographic, seasonal, and geographic patterns for 1999-2004. During this period, an average of 439 persons died annually from unintentional, non-fire-related CO poisoning, and the national average annual death rate was 1.5 per million persons. However, rates varied by demographic subgroup, month of the year, and state. Rates were highest among adults aged > or =65 years, men, non-Hispanic whites, and non-Hispanic blacks. The average number of deaths was highest during January. Among the states, Nebraska had the highest reliable CO mortality rate. These findings indicate that improved population-based prevention measures, including educating the public about the dangers of CO exposure, are needed at the state and national levels.