Circulating platelet aggregates formed in vivo were serially measured, and platelet-aggregation thresholds were determined in vitro in 82 patients with acute cerebral ischaemia. The percentage of aggregated platelets was increased in 53 patients with completed stroke (30.9% +/- 2.0) and in 29 patients with transient ischaemic attacks (34.1% +/- 2.3), all studied within 10 days of the acute event. These values were higher (P less than 0.001) than levels of aggregated platelets in 30 patients with non-vascular neurological disease (16.8% +/- 2.3). The percentage of aggregated platelets returned to normal 10 days to 6 wk after acute cerebral ischaemia. Aspirin and dipyridamole did not affect either the increase in or subsequent normalisation of circulating-platelet-aggregate levels in these patients. Platelet-aggregation sensitivity to adenosine diphosphate and adrenaline was also increased in patients with acute cerebral ischaemia, but this abnormally resolved during convalescence. Platelet activation is abnormal in acute cerebral ischaemia but usually returns to normal with or without anti-platelet therapy. This activation of platelets may contribute to the clinical manifestations of occlusive vascular disease.