Atrial tachyarrhythmias are important complications occurring in more than 8% of acute myocardial infarctions (AMI). Atrial fibrillation (AFi) during the early phase of AMI is caused by atrial ischaemia, atrial distension due to the left ventricular failure or significant diastolic left ventricular dysfunction. AFi in patients with inferior and posterior AMI indicates at least two vessel coronary diseases, a circumflex coronary artery (CX) occlusion before taking off of the left atrial branches as well as significant stenosis or occlusion of the right coronary artery (RCA). In this article the case of a 67-year-old woman with an acute infero-posterior AMI is described. AMI was complicated with a left heart failure, acute AFi with tachyarrhythmia, transient arterial hypotension and ischaemic mitral regurgitation. Emergency coronary angiography disclosed occlusion of the CX, myocardial infarct related artery, and significant stenoses of the RCA. After opening the occluded CX during the PTCA, AFi with a tachyarrhythmia of 160 beats per minute (bpm) immediately converted into a sinus rhythm with 80 bpm, followed by a normalization of blood pressure and cardiac recompensation. Our case report supports the opinion that AFi in patients with inferior and posterior AMI indicates at least a two-vessel coronary disease. Reopening of the occluded atrial coronary branches during urgent medical treatment was casual and effective treatment of both ischaemic heart disease and consequent AFi.