Carbon monoxide (CO) is responsible for more poisoning fatalities each year than any other toxic agent. The often insidious nature of the symptom progression and its ability to imitate many common illnesses may result in the failure to diagnose a potentially fatal outcome. CO detectors equipped with an audible alarm can alert potential victims of CO poisoning before toxic sequelae develop. A study was conducted in which all calls to 911 concerning a CO detector in alarm or regarding possible CO poisoning were investigated by a paramedic crew; 101 possible CO exposures were investigated. CO detectors with audible alarms were the genesis of 59.4% of the calls. Detectable CO levels were found in 69.3% of the investigations, and 80% of the homes with detectors had verifiable CO concentrations. The mean CO concentration in homes with detectors was 18.6 ppm, compared with 96.6 ppm when no detector was available; 63.4% of the victims with no alarm were symptomatic, compared with 13.3% of victims with alarms. CO detectors with audible alarms were effective in alerting the potential victims of CO poisoning to its presence. Persons with CO detectors were less likely to become symptomatic from a CO exposure than those who did not have CO detectors.