In 1350 patients with suspected acute myocardial infarction, serum potassium levels during the first 3 days in hospital was correlated to clinical history and clinical course. A higher incidence of hypokalemia was observed in women, in patients with hypertension, and in those on chronic diuretic treatment. Patients with anterior infarction had a higher incidence of hypokalemia than those with inferior infarction, as did patients with large as compared with small infarcts. No clear difference was observed between patients whose infarction was confirmed and those in whom the diagnosis was not confirmed. Independent predictors for hypokalemia were treatment with diuretics before admission to hospital, infarct size, and female sex. Hypokalemia during the first 3 days of hospitalization was associated with the occurrence of severe ventricular arrhythmias during hospitalization, but not with survival during a 5-year follow-up.