Context It is unclear if people who develop stress hyperglycaemia have underlying abnormal glucose metabolism, an exaggerated hormonal response to stress, or both. Similarly, it is unknown whether stress hyperglycaemia predicts future glucose intolerance. Objective To determine the relationship between illness severity and plasma cortisol concentration with the degree of hyperglycaemia in subjects experiencing acute myocardial infarction (AMI), and their later glucose metabolic status. Patients and design This analysis comprised 55 patients from the HI-5 Study - a randomized control trial of insulin-glucose infusion therapy for AMI patients with hyperglycaemia. Main outcome measures Blood glucose level (BGL) as well as total and free cortisol levels on admission were measured. Patients not previously known to have diabetes were assessed for abnormal glucose metabolism following discharge. Results Patients with ST segment-elevation myocardial infarction (STEMI) and higher peak creatinine kinase level had a higher mean admission total and free cortisol level. As many as 38·5% of subjects were found to have newly diagnosed glucose intolerance at follow up. After multiple logistic regression, admission BGL was found to be a positive predictor (P = 0·027) whereas total cortisol level (P = 0·01) was a negative predictor for glucose intolerance. Conclusions Both the level of hyperglycaemia and cortisol levels on admission are predictive for the subsequent abnormal glucose tolerance development in hyperglycaemic AMI patients. Hyperglycaemia in patients who are more unwell (i.e. higher cortisol) reflects the stressed state rather than underlying glucose intolerance. Conversely, if the patient is less sick (i.e. lower cortisol), hyperglycaemia is more likely to reflect underlying glucose intolerance.