A high prevalence of newly detected diabetes mellitus (DM) and impaired glucose tolerance (IGT) has been reported in patients with acute myocardial infarction (AMI) and no previous diagnosis of DM. However, the prevalence of newly detected DM is grossly underestimated by using fasting plasma glucose (FPG). We determined the prevalence of DM and IGT in patients post-AMI from our local mixed ethnicity population, and evaluated the usefulness of oral glucose tolerance testing in such patients. All non-diabetic subjects admitted with AMI underwent a standardised oral glucose tolerance test (OGTT) with 75 g glucose load predischarge in our institution. Fasting and 2-h postchallenge plasma glucose levels were recorded, in addition to admission plasma glucose, serum cholesterol, triglycerides, HDL cholesterol and haemoglobin A1(C)levels. We studied 61 patients [38 (62%) male; mean (SD) age, 64 (12.5) years], of whom 70% were white European and 30% South Asians. Mean (SD) plasma glucose concentration on admission was 6.9 (1.7; range, 5.8-8.1) mmol/l. Newly diagnosed DM and IGT were detected in 31% (95% CI 10-52) and 33% (95% CI 12-53) of patients respectively. Of those with newly detected diabetes only 32% (95% CI 0-69) had FPG > 6.1 mmol/l. The 12 month major adverse cardiac event rate was 4.5%, 15% and 32% in those with normal glucose tolerance, IGT and DM respectively. Previously undiagnosed DM and IGT in patients with AMI is common. The false reassurance of a normal FPG denies a significant proportion of undiagnosed diabetics the chance of early treatment. The importance of OGTT in the diagnostic work up of this vulnerable high-risk group cannot be over-emphasised.