Carbon monoxide (CO) poisoning is an important health problem. Cardiac abnormalities may occur in patients with CO poisoning; however, the severity and duration of cardiac abnormalities are not well known. In this study, cardiac structures and function in CO poisoning were evaluated prospectively. Twenty patients were enrolled in the study. Echocardiographic examination was performed in all patients on admission, at 24 hours, and within the first week. B-type natriuretic peptide and carboxyhemoglobin levels were measured. Patients with increased cardiac markers underwent coronary angiography. Cardiac markers were high in 6 patients. Patients with high cardiac markers had significantly higher carboxyhemoglobin levels and longer exposure to CO. Left ventricular ejection fraction (LVEF) was <45% in 8 patients (group I) on admission and >55% in 7 patients in group I 24 hours after echocardiography. A significant negative correlation was found between B-type natriuretic peptide and LVEF on admission (r = -0.586, p <0.01). The decrease in LVEF was also negatively correlated with carboxyhemoglobin level and CO exposure duration. All angiograms showed normal coronary arteries. In conclusion, despite normal coronary arteries, myocardial dysfunction may occur in patients with CO poisoning. LV systolic function might be normal or mildly to severely impaired. However, most of the myocardial dysfunction dissipates at 24 hours in patients with CO poisoning.