Serum potassium concentration was estimated on admission to hospital in 289 women and 785 men with acute myocardial infarction. The proportion of women in potassium subgroups was inversely related to serum potassium concentration, increasing from 8% at serum potassium greater than or equal to 5.2 mmol/l to 58% at less than or equal to 3 mmol/l. The frequency of diuretic therapy was also higher in women (35%) than in men (23%). The mortality rate was high at 3 months in patients with one or more arrhythmias (atrioventricular block grade 2, complete heart block, bundle branch block, atrial fibrillation, premature ventricular contractions, ventricular tachycardia) detected by conventional methods during the first 48 hours after admission. Hypokalemia (serum potassium less than or equal to 3.5 mmol/l) did not significantly predict increased occurrence of any of these arrhythmias. Small inhomogeneities of arrhythmias between the potassium groups may have been caused by digitalis therapy prior to admission. Hypokalemia on admission did not predict altered prognosis during the first 3 months.