Prevalence of carbon monoxide (CO) alarm usage in localities where they are not required is poorly defined and the reasons for failing to have a home CO alarm have never been described. In this study, the authors conducted a computer-based survey among employees of similar major medical centers in Seattle, Washington, and Salt Lake City, Utah. Questions were asked about the prevalence of use of residential smoke and CO alarms with regard to home style and structure, ownership status, and energy use. Respondents not using home CO detectors were asked the reasons. Among 1,351 individuals participating in the survey, 98% reported residential use of smoke alarms, while only 51% used CO alarms. CO alarm use was more common among residents of Utah than Washington, among home owners than renters, and among those with single family homes rather than other styles. Reasons for failure to use CO alarms related largely to lack of knowledge about the devices and motivation.