STUDY OBJECTIVE - The objective was to measure platelet function in vivo (as cutaneous bleeding time), the role of catecholamines, and the effect of inhibiting thromboxane synthesis on bleeding time, in patients with myocardial infarction, unstable angina, and non-cardiac chest pain. DESIGN - Haemotological variables and plasma catecholamines were compared between patient diagnostic groups using the Kruskal-Wallis test, Conover's multiple comparison test, and Wilcoxon paired rank sum test. PATIENTS - 49 patients entered the study and 45 were assigned to three groups: myocardial infarction (n = 26), unstable angina (n = 9), and non-coronary chest pain (control) (n = 10). There were no significant differences between groups for age or sex. Patients with myocardial infarction smoked more than others. MEASUREMENTS and RESULTS - Compared to the controls, bleeding time in patients with myocardial infarction was shortened, while in unstable angina it was normal. Plasma adrenaline and noradrenaline concentrations were higher in the myocardial infarction group than in the unstable angina and control groups, but were not correlated with bleeding time. Bleeding time was remeasured 2 h after ingestion of 300 mg aspirin and increased in all subjects, especially in those with myocardial infarction, but it remained significantly shorter in the infarct group than in the comparison groups. Plasma adrenaline was inversely correlated with the bleeding time after aspirin in the infarct group. CONCLUSIONS - The shortened bleeding time may be an indicator of an increased prethrombotic tendency present in patients with myocardial infarction but not in those with unstable angina. The effect appears to be mediated by both thromboxane A2 and adrenaline.