In a retrospective review of patients admitted to a coronary care unit, 809 patients had a definite myocardial infarction and plasma potassium was measured within 48 hr. The mean (+/- s.d.) plasma potassium was 4.04 (+/- 0.56) mmol/litre. A potassium concentration of less than 4.0 mmol/litre was found in 383 patients, of whom 14 had ventricular fibrillation. A potassium concentration of 4.0 mmol/litre or greater was found in 426 patients, of whom 3 had ventricular fibrillation, a difference significant at the 0.4% level (chi 2 test). Preceding diuretic therapy did not appear to be relevant and there appeared to be no other difference between the two groups. The maintenance of a high-normal plasma potassium, or the prevention of its reduction, in patients with an acute myocardial infarction may be important.