We evaluated leucocyte aggregation in 26 patients with ischemic stroke and in 10 patients with transient ischemic attacks (TIA), previously untreated, within 24 h from the onset of symptoms. The evaluation was also performed in 30 healthy subjects matched for age and sex. Leucocyte aggregation was significantly higher in patients than in controls (p < 0.01 post hoc Tukey test). Within patients, those with stroke showed a significantly higher aggregation than those with TIA (p = 0.01 post hoc Tukey test). Moreover, stroke patients with the poorest outcome showed significantly higher values of leucocyte aggregation. These results indicate an involvement of leucocytes in cerebral ischemia and suggest that changes in their aggregability may play a role in the evolution of the disease.