Circulating platelets are susceptible to various stimuli in vessels, and a certain portion of platelets may thereby form native microaggregates (NMAs), which can be visualized by a bother scanning electron microscopy (SEM). We developed an easy method to evaluate the existence and extent of NMAs and the subsequent spontaneous platelet aggregation (SPA) induced by not any agonist but stirring force with a novel platelet aggregometer (PA-200; Kowa, Tsukuba, Japan) employing a particle-counting method based on laser light scattering. In this system, platelet-rich plasma obtained by a delicate centrifugation was infused in a cuvette to replace the content and to avoid stirring shear. An insertion attachment was designed in a coaxial shape to maintain a constant sample volume. Our data indicated that NMAs, tentatively defined as a "small" division by their light scattering (25-400 of the intensity threshold), could be detected by not only a complicated SEM but also by our simple system with the PA-200 in a few minutes. No correlation was found in the comparison of the existence of NMAs and the SPA generation, in aliquot platelets, respectively. These results suggested that NMAs and SPA might be mutually different cluster and have meanings as representations of platelet status.