The threshold concentrations of arachidonic acid (AA) required to induce platelet aggregation were measured in platelet-rich plasma (PRP) from patients with acute ischaemic heart disease and healthy controls. The analytical precision of the test was very good (coefficient of variation 0–4%). Analytical accuracy was evaluated by comparing the results with threshold values for collagen-induced platelet aggregation, and a significant correlation was found (r = 0.56; p<0.01). When comparing the serum levels of thromboxane B2to threshold values for AA-induced platelet aggregation an inverse relationship was found (r = -0.37; p<0.01). In the clinical study significantly increased aggregability to AA was seen in patients with unstable angina pectoris (n = 13) compared to patients with stable angina pectoris (n = 14), (p<0.01), and both groups had hyperaggregating platelets compared to healthy controls (n = 27), (p<0.01). The patients with acute myocardial infarction (n = 10) had nearly normoaggregating platelets for the first 2–3 days after admission, but after a week and at day 14 their platelets showed significant hyperaggregability compared to healthy controls (p<0.01). Thus studies of AA-induced aggregation ex vivo suggest that patients with unstable angina pectoris and acute myocardial infarction, in whom coronary thrombus is frequently present, have increased platelet aggregability compared to patients with stable angina pectoris and healthy controls.