In the present study, 408 patients with acute myocardial infarction were included. The serum concentration of potassium was assessed on admission. Episodes of ventricular fibrillation and/or ventricular tachycardia within the following 6 hours were registered. A significant positive correlation between hypokalemia and the incidence of malignant ventricular arrhythmias was demonstrated. Ongoing treatment with diuretics at the time of admission did not appear to be of any significance for the development of ventricular fibrillation or ventricular tachycardia. Out of 100 hypokalemic patients, only 33 were treated with diuretics. The main reason for hypokalemia in the early phase of an acute myocardial infarction is most likely an activation of the sympathetic nervous system leading to an influx of potassium from the extracellular to the intracellular body fluid compartment.