Background　Studies　have　shown　that　platelet-leukocyte　aggregates　(PLA)　are　sensitive　to　platelet　activation　which　might　exist　before　the　onset　of　cerebral　infarction.　In　this　study,　we　investigated　the　formation　of　PLA　in　patients　with　cerebral　infarction　and　the　effects　of　antiplatelet　agents　on　PLA.　Methods　The　level　of　soluble　P-selectin,　C-reaction　protein,　platelet　aggregation　rate　and　leukocyte-platelet　aggregations　were　measured　in　40　patients　with　acute　cerebral　infarction　and　20　normal　controls.　The　40　patients　were　randomly　assigned　to　two　treatment　groups:　aspirin　group　(n　=　20)　and　clopidogrel　group　(n　=　20).　Both　groups　were　monitored　for　Scandinavian　stroke　scale　(SNSS),　soluble　P-selectin,　serum　C-reaction　protein,　platelet　aggregation　rate　and　PLA　before　and　after　the　treatment.　Flow　cytometry　was　used　to　detect　the　levels　of　PLA　in　the　blood.　Results　The　percentage　of　platelet-monocyte　aggregates　(PMA)　in　patients　with　cerebral　infarction　was　significantly　increased　compared　with　the　controls　(P　＜　0.001),　which　was　positively　correlated　to　soluble　P-selectin,　C-reaction　protein　and　platelet　aggregation　rate　(P　＜　0.05).　After　the　treatment,　the　levels　of　PMA　and　platelet　aggregation　rate　were　decreased　in　both　groups　(P　＜　0.05).　The　level　of　PMA　and　platelet　aggregation　rate　in　the　clopidogrel　group　was　significantly　lower　than　that　in　the　aspirin　group　(P　＜　0.05).　Conclusions　PMA　are　a　sensitive　biomarker　to　platelet　activation　in　patients　with　cerebral　infarction.　In　addition,　although　both　aspirin　and　clopidogrel　lowered　the　level　of　PMA,　clopidogrel　is　a　more　effective　treatment　than　aspirin　in　inhibiting　platelet　activation.