OBJECTIVE--To determine whether diabetic patients admitted with acute myocardial infarction have impaired fibrinolytic activity due to raised plasminogen activator inhibitor compared with non-diabetic patients. SETTING--A district general hospital. PATIENTS--90 non-diabetic and 38 diabetic patients admitted with acute myocardial infarction. RESULTS--Both plasminogen activator inhibitor activity and antigen were significantly higher in diabetic than in non-diabetic patients (24.7 (6.8) v 18.5 (6.8) AU/ml; p = 0.0001 and 64.2 (range 13.1 to 328.8) v 38.5 (range 10.9 to 173.7 ng/ml; z = 3.3; p = 0.0008) with a positive correlation between activity and antigen (rs = 0.51; p = 0.0001). In both groups, activity and antigen concentrations were significantly higher than in diabetic and non-diabetic subjects without coronary artery disease (p = 0.002 to 0.0001 for each comparison). Plasminogen activator inhibitor activity correlated significantly with admission plasma glucose (r = 0.32; p = 0.0001), glycated haemoglobin (r = 0.32; p = 0.0001), admission plasma insulin (rs = 0.48; p = 0.001), and Killip grade of heart failure both on admission (rs = 0.27; p = 0.001) and on discharge (rs = 0.22; p = 0.006), but not with cumulative creatine kinase MB isoenzyme release (rs = -0.08). There were similar but weaker correlations between tissue plasminogen activator antigen and admission plasma glucose, glycated haemoglobin, and insulin. In 18 patients (12 non-diabetic and six diabetic) plasminogen activator inhibitor activity was measured between six and 12 months (8.3 (1.6)) after the acute infarct and remained similar to activity on admission (24.8 (1.9) AU/ml (NS) for diabetic and 17.9 (6.9) AU/ml (NS) for non-diabetic patients) and was still significantly higher in diabetic than in non-diabetic patients (p = 0.007). CONCLUSION--These results show that diabetic patients have higher plasminogen activator inhibitor activity than non-diabetic patients both on admission with acute myocardial infarction and at follow up six to 12 months later. Raised plasminogen activator inhibitor activity may predispose diabetic patients to myocardial infarction and may also impair pharmacological and spontaneous reperfusion after acute myocardial infarction thus contributing to the poor outcome in these subjects.