Objective:To examine which recumbent position can lead to the highest vagal modulation in patients during the acute phase of myocardial infarction.Design:Descriptive study.Setting:Intensive care unit in a medical center.Patients:A total of 52 patients admitted to the intensive care unit because of acute myocardial infarction (AMI), 41 patients with coronary artery disease (CAD), and 28 patients with patent coronary arteriogram.Interventions:None.Measurements and Main Results:Heart rate variability analysis was performed in patients with AMI, patients with CAD, and patent coronary controls in supine, left lateral decubitus, and right lateral decubitus positions in random order. In the right lateral decubitus position, the vagal modulation was the highest and the sympathetic modulation was the lowest among three recumbent positions in three groups of patients. When the position was changed from supine to right lateral decubitus, the increase in vagal modulation was greater in patients who had more severely depressed vagal modulation in the supine position and the rate of increase was the greatest in patients with AMI, followed by patients with CAD and patent coronary controls. Detailed analysis showed that the vagal enhancing and sympathetic suppression effect of the right lateral decubitus position applied to patients with Q wave myocardial infarctions.Conclusions:The right lateral decubitus position can lead to the highest vagal modulation and the lowest sympathetic modulation among three recumbent positions in patients with Q wave myocardial infarction. The right lateral decubitus position can be used as an effective vagal enhancer in patients with Q wave myocardial infarction but without severe bradycardia or atrioventricular block.