A pilot study was carried out in 22 non-diabetic patients admitted to hospital within 12 hours of acute myocardial infarction to assess the plasma glucose level on admission as an index of infarct size, morbidity and prognosis. Blood free fatty acid, insulin and potassium concentrations were also measured. Mean enzymatically measured infarct size index was three times larger (p less than 0.005) and the incidence of large infarcts was significantly greater (p less than 0.05) in patients with high (greater than 7.5 mMol/L) than in patients with low (less than 7.5 mMol/L) admission glucose concentrations. Patients in the former group also had higher mean peak free fatty acid concentrations (p less than 0.05). Patients who died within six months of their first infarcts had higher initial glucose values than those who survived longer. Plasma glucose concentrations measured within 12 hours of the onset of symptoms of acute myocardial infarction can be used to differentiate those with large from those with small infarcts in non-diabetic patients.