Hyperglycaemia occurring at admission in patients with suspected acute myocardial infarction is generally held to represent stress hyperglycaemia. 26 patients, not previously known to be diabetic, had blood glucose values greater than or equal to 10 mmol/l on admission to a coronary care unit. 16 survived for 2 months at which time a 75 g oral glucose tolerance test (OGTT) showed diabetes in 10 (63%) and impaired glucose tolerance in 1 (WHO criteria). All those with abnormal glucose tolerance at 2 months had had raised glycosylated haemoglobin (HbA1) (greater than 7.5%) on admission, indicating pre-existing diabetes. All those with a HbA1 level over 8% had abnormal glucose tolerance. 7 of the 10 who died or did not have an OGTT also had raised HbA1 at admission. An admission blood glucose greater than or equal to 10 mmol/l in patients with severe chest pain is more likely to indicate previously undiagnosed diabetes than "stress" hyperglycaemia. There is no evidence that myocardial infarction precipitates diabetes. The glycosylated haemoglobin concentration can be used to distinguish between stress hyperglycaemia and hyperglycaemia caused by diabetes.