The records of 151 patients entering a coronary care unit and subsequently diagnosed as having an acute myocardial infarction were reviewed. The prevalence of hypokalemia, its relationship to diuretic treatment and the development of either ventricular tachycardia or ventricular fibrillation were studied. At admission, 14% of patients were hypokalemic. The presence of hypokalemia was related to previous diuretic therapy. Twenty-three percent of patients receiving diuretics as compared to 7% of patients not taking diuretics had a serum potassium of 3.5 mEq/L or less. Thirty-seven patients experienced either ventricular tachycardia or ventricular fibrillation. The presence of hypokalemia was associated with an increased frequency of both of these arrhythmias. Sixty-seven percent of patients with a serum potassium of less than 3.1 mEq/L had these serious ventricular arrhythmias compared to 40% of patients with a serum potassium between 3.1 and 3.5 mEq/L and 20% of normokalemic patients. The prevalence of premature ventricular beats was not correlated with the presence of hypokalemia. We conclude that hypokalemia is not only a common problem in patients with acute myocardial infarction but a clinically significant factor in the development of life threatening arrhythmias. Primary prevention of hypokalemia and its prompt treatment are indicated in these patients.