In patients with acute coronary syndrome (ACS), the presence of atrial fibrillation (AF) results in worse inpatient outcomes than in those without AF Two electrocardiographic markers, maximum P wave duration (P-maximum) and P wave dispersion P-dispersion), have been assessed because they reflect conduction abnormalities in patients with paroxysmal AF. beta blockers are known to have beneficial effects in patients with ACS. This prospective study was conducted to investigate whether early intravenous (IV) metoprolol injection acutely decreases P-maximum and P-dispersion in patients with ACS. This study involved 100 consecutive patients with ACS who were divided into 2 groups according to whether or not they received early IV metoprolol. Group 1 consisted of 19 patients who received IV metoprolol within 3 h after onset of symptoms, and group 2 consisted of 81 patients who did not receive IV metoprolol within 3 h after symptom onset because of late admission. P-maximum and P-dispersion, were measured on admission and again at 2 h after admission. Two-dimensional echocardiographic examination was also performed. For patients who received early IV metoprolol, P-maximum and P-dispersion. measured 2 h after admission, were shorter than values at admission (P<.001). Conversely, P-maximum and P-dispersion measured 2 h after admission, did not differ significantly from values at admission in patients who did not receive early IV metoprolol (P=.292 and P=.236, respectively). IV administration of metoprolol reduced values for P-maximum and P-dispersion measured 2 h after admission, among patients with ACS who were admitted within 3 h after onset of symptoms.