Background: Not only platelet but also leukocyte plays a crucial role in the pathophysiology of atherosclerosis and arterial thrombosis which is major cause of ischemic stroke. Macrophage-1 antigen (Mac-1) is up-regulated on activated leukocytes and has been shown to be a crucial regulator of leukocyte recruitment by promoting leukocyte firm attachment to adherent platelets and fibrionogens in injured artery or atheromatous plaque. To clarify role of leukocyte activation in arterial thrombosis, we observed Mac-1 expression of leukocyte with P-selectin expression of platelet and circulating level monocyte-platelet aggregates, both of which are well known platelet activation markers. Methods: Mac-1 expression of leukocyte, P-selectin expression of platelet and circulating level of monocyte-platelet aggregates were analyzed in 28 patients at acute and convalescent phase of stroke, and in 21 control subjects. Results: Mac-1 expression of leukocyte was higher in acute phase of stroke than convalescent phase of stroke (P=0.032) and control group (P=0.045). Pselectin expression of platelet and circulating level of monocyte-platelet aggregates revealed same pattern as Mac-1 expression of leukocyte, and all these three showed a positive correlation. Conclusion: These findings support the role of leukocyte activation and platelet-leukocyte interaction in arterial thrombosis. Mac-1 of leukocyte and P-selectin of platelet may be a new therapeutic target of arterial thrombosis including ischemic stroke.