Coronary thrombosis has been implicated in the pathogenesis of acute coronary syndromes, and platelet activation plays a pivotal role in the pathogenesis of coronary thrombus. A new platelet aggregometer using a laserlight scattering beam was trialled for assessment of platelet aggregation. Platelet aggregability, especially small-sized platelet aggregates, was investigated on admission using the laser-light scattering method and again after treatment in 23 patients with acute coronary syndromes. The platelet aggregability in 14 patients with stable exertional angina and in 14 control subjects was also examined. On admission, the number of small- and medium-sized platelet aggregates in the acute coronary syndromes group was significantly greater than in the stable exertional angina group or control group. However, the number of large-sized platelet aggregates on admission was not increased in the acute coronary syndromes group. Furthermore, the number of small- and medium-sized platelet aggregates decreased significantly after treatment in the acute coronary syndromes group. The increased number of small-sized platelet aggregates may sensitively reflect attacks of thrombosis in patients suffering acute coronary syndromes.