Several studies over the last decade have linked hyperglycaemia on hospital admission with subsequent mortality risk. The evidence is strongest for patients with myocardial infarction or acute coronary syndromes, but evidence also links hyperglycaemia with mortality from stroke and other medical illnesses. The effect seems independent of a previous diagnosis of diabetes mellitus; indeed, some studies suggest that mortality may be higher in patients with hyperglycaemia and no previous diabetes diagnosis compared with known diabetic patients. The effect on outcome of therapeutically lowering blood glucose levels has been considered in a small number of studies, but so far the results are conflicting. Further work is needed, focusing on more standardized surveys--previous studies vary in their use of blood or plasma, as well as cut-off levels for hyperglycaemia--and larger intervention studies.