Study objectives: Carbon monoxide (CO) poisoning is not infrequently treated in the emergency department (ED). In that the heart is one target organ affected in CO ingestions, it follows that the ECG is a potentially important investigation in these patients. We review the ECG findings in patients with CO poisoning with correlation to the clinical presentation. Methods: Consecutive ED patients with CO poisoning identified by toxicology service medical records represent the study group. Criteria for inclusion included pathologically abnormal CO serum level and ECG performed within 6 hours of presentation. Medical records and ECGs were reviewed; the ECG reviewers were blinded to the other medical data. Three emergency physicians reviewed the ECGs for rhythm and morphologic features. Results: Twenty-six patients met entry criteria and were used for data analysis. The mean age of the patients was 36.4 years. The ECG demonstrated abnormality in 15 cases (56.7%). Mean serum CO and troponin values were 25.0 mg/dL and 0.60 mg/dL, respectively. ECG findings included rhythm disturbances (sinus bradycardia, 2 (7.7%)] and sinus tachycardia, 4 (15.4%)]) and morphologic abnormalities (prolonged PR interval, 0; widened QRS complex, 4 [15.4%]; prolonged QT interval 2 [7.7%]; ST segment depression, 4 [15.4%]; ST-segment elevation, 3 [11.5%]; and T wave inversion, 10 [38.5%]). Morphologic ECG findings involving the ST segment and T wave more often occurred in the anterior distribution (P<.0001). Patient age, increasing serum CO level, and increasing serum troponin values were all associated with greater degrees of ECG abnormality (P<.0001 for all comparisons). Conclusion: ECG abnormality is frequent in patients poisoned with CO. Such abnormalities most often involved the ST segment and T wave, features associated with acute coronary ischemia. Dysrhythmias were rare and, if encountered, benign in nature. An abnormal ECG was more often found in patients with increased age and higher serum CO and troponin values.