SummaryA number of platelet function tests (retention tests, electrophoretic platelet mobility, platelet turnover, induced and spontaneous aggregation) have been used to confirm the role of enhanced thrombocyte adhesiveness or aggregation as risk factors for progressive atherosclerosis and its complications, with conflicting results.«Spontaneous»platelet aggregation using PAT I has been studied in a great number of healthy persons and in patients with a wide variety of internal diseases, especially vascular and thrombo-embolic disease over the past years. The incidence of enhanced platelet aggregation in healthy subjects is increasing with age. In diabetes mellitus the incidence of enhanced platelet aggregation was greater than in age matched controls, and there was another 10% increase in diabetes of more than five years’duration. This finding is in good correlation with the progression of diabetic angiopathy with the duration of the underlying disease. A very high incidence of enhanced platelet aggregation and only rare cases with normal (=missing) aggregation were observed in patientspriorto thromboembolic disease like myocardial infarction and arterial or venous thrombosis.Studies with a recently developed aggregometer revealed the importance of time dependant changes of platelet morphology (primary shape changes) upon the shape of aggregation curves in spontaneous aggregation (PAT III) and ADP-, collagen- or epinephrine-induced aggregation. If the various aggregation tests are being performed at the right time after blood sampling, they will yield very similar results.