Platelet aggregation is usually measured by a spectroscopic method using an aggregometer. However, when the process of platelet aggregation is followed by the changes in light transmission of platelet-rich plasma (PRP), factors such as platelet deformation or changes in plasma permeability must be taken into consideration. Thus, a better method was developed in our laboratory for measuring platelet aggregation by chronologically counting non-aggregated single platelets, one of the simplest parameters of the aggregation process, using PRP to which adenosine diphosphate (ADP) was added.1,2In the present study, we attempted to further evaluate platelet aggregability by measuring the velocity of platelet aggregation in different subjects using our single platelet counting method.