We evaluated simultaneously leukocyte and platelet aggregation in 40 untreated patients soon after atherothrombotic stroke and in 40 healthy controls matched for age, sex and risk factors. Both leukocyte and platelet aggregation were significantly increased in patients relative to controls (p < 0.01 and p < 0.02, respectively). However, whereas leukocyte aggregation was positive in a significantly higher proportion of patients with subsequent major neurological deficit compared to those with minor deficit (71% vs. 19%; p = 0.04), platelet aggregation was similar in the two patient groups. These data suggest different patterns of activation of leukocyte and platelet aggregation in patients with ischemic stroke. This finding may have pathophysiological and clinical implications.