This study was conducted to determine the annual number of emergency department (ED) visits and rate of hyperbaric oxygen (HBO2) treatment for carbon monoxide (CO) poisoning in Washington, Idaho, and Montana. All hospital emergency departments and hyperbaric treatment facilities in the region were surveyed by mail and telephone regarding their patient treatment experience for calendar year 1994. Results demonstrated that there were approximately 2.51 million total ED visits in 1994 in the three states studied. Among these, an estimated 1,325 individuals were seen with carbon monoxide poisoning (52.9 CO cases per 100,000 ED visits; 18.1 CO cases per 100,000 population). A total of 91 patients were treated with HBO2, yielding an HBO2 treatment rate of 6.9% of those evaluated in EDs. Extrapolating these figures to the US population suggests that the number of individuals seeking emergency medical care for CO poisoning is much greater than is commonly quoted. Even after correcting for the known increased rate of CO poisoning in the Pacific Northwest, the incidence of nonfatal poisoning appears to be significantly higher than may be appreciated from previous reports.