Because cases of unrecognized carbon monoxide (CO) poisoning have been described among patients admitted to the hospital with other diagnoses, screening hospital admissions with carboxyhemoglobin testing has the potential for preventing morbidity among patients as well as among their cohabitants. Carboxyhemoglobin levels were obtained on 753 patients admitted to the hospital from the emergency department over a 3-month period during the winter. Patients in whom CO poisoning was diagnosed in the emergency department prior to admission were excluded. The mean carboxyhemoglobin level was 2.52% +/- 1.85%; there was no significant difference in mean carboxyhemoglobin among patients with medical, surgical, neurological, and psychiatric admission diagnoses (F = 1.17; df = 3,746; P = .32). Two patients (0.3%; 95% confidence limits, 0.04% to 1.1%) from the entire admission cohort, and one of 20 patients (5%; 95% confidence limits, 0.3% to 26.9%) admitted with seizures, had carboxyhemoglobin levels greater than 10%. The carboxyhemoglobin levels of the two patients were only marginally elevated, with levels of 10.9% and 11.3%. The cost of the carboxyhemoglobin screening program was $2.26 per patient result, or approximately $2,100 over a 3-month winter heating season. A program for screening emergency department admissions with carboxyhemoglobin testing, although feasible in terms of cost, detected few cases of unrecognized CO poisoning.