Platelet aggregation and release studies aid in the diagnosis of bleeding disorders. However, variability in normal individuals has not been established. We evaluated the variation in platelet aggregation and release testing over a period of 2 years with 59 observations among 5 subjects. In addition, the intra-run variation for specific agonists and the adenosine triphosphate standard was determined. The average coefficients of variation (CV) for aggregation with each agonist, except the lower 2 concentrations of ristocetin, were less than 17%. The CV for the 1.0 mg/mL ristocetin aggregation for each subject ranged from 42% to 160%. The average CVs for all the release tests were greater than 30%; with 5 and 10 muM adenosine diphosphate, 56% and 42%, respectively. Decreased adenosine triphosphate release with epinephrine as the sole abnormality occurred in only one of the 59 separate studies. The significance of an abnormal result in initial testing of a patient will depend on understanding this variation.