BACKGROUND: The purpose of this study was to evaluate the time course of PAC-1 and the P-selectin expression after the initiation of antiplatelets in acute ischemic stroke. METHODS: We serially measured PAC-1 and the P-selectin expression level using flow cytometry immediately before and at: one day, seven days, and one month after starting antiplatelets in 25 patients with acute cerebral infarctions. RESULTS: The P-selectin expression increased initially (p<0.05) and then from the seventh day, it began to decrease continually (post-hoc analysis, p<0.05). However, the PAC-1 expression did not reveal significant alterations during the follow-up period of one month. CONCLUSIONS: Although the P-selectin expression rapidly declined, the PAC-1 expression remained elevated for one month. Our results suggest that the P-selectin may be useful in monitoring platelet inhibition during the early period after cerebral infarction.