Platelet function testing consisting of platelet aggregation and secretion often is requested in the clinical evaluation of patients with bleeding problems. At present, there are no uniform clinical laboratory standards for the performance or interpretation of these studies. The present report describes one laboratory's methods and interpretations of platelet aggregation and secretion studies of platelet-rich plasma using each of the common platelet agonists. Diagnostic categories for the evaluation of the platelet function testing are presented. The diagnostic categories then are applied to the evaluation of 61 patients referred to our medical center for these studies. The aims of this report are to present clinical platelet aggregation and secretion studies and to provide a working schema to evaluate these results. Our intent is to stimulate interest in the development of professional guidelines for platelet function testing in the clinical laboratory.