Light transmission aggregometry (LTA) is the gold standard for the study of patients with defects of platelet function. Use of LTA in clinical practice for predicting the risk of thrombosis or monitoring the pharmacologic effects of antiplatelet agents should be discouraged, because not only is the monitoring of treatment with antiplatelet agents (with any laboratory test) not indicated at present, but also the lack of standardization of the technique for LTA makes it additionally unsuitable for this purpose. The need for standardization of LTA has recently been emphasized by the results of four surveys, which showed that there is a wide variation in the methodology used worldwide. A modification of the traditional LTA is the lumiaggregometer, which measures platelet secretion in parallel with platelet aggregation. This technique is probably preferable to traditional LTA in the diagnostic workup of patients with inherited defects of platelet function, because it is more sensitive to the most common disorders, which are characterized by abnormalities of platelet secretion. LTA (or lumiaggregometry) is useful as a first screening test of patients with the clinical suspicion of defects of platelet function because it helps to provide an interim diagnostic hypothesis, which can then be confirmed or discounted using appropriate and specific tests.