Adenosine diphosphate-induced platelet aggregation and associated thromboxane B2 release were studied in 52 patients with subarachnoid hemorrhage (SAH) in order to detect a possible association between altered platelet function and development of cerebral ischemic complications after SAH. Compared to the values on admission, the patients showed significantly increased platelet aggregability (p less than 0.05) and thromboxane release (p less than 0.001) 1 to 2 weeks after SAH. The highest values of thromboxane release were seen in patients who deteriorated due to delayed cerebral ischemia with a permanent neurological deficit. Thromboxane release was significantly higher (p less than 0.05) before the onset of severe delayed ischemia in six patients with preoperative ischemia compared to the patients without delayed ischemia. In five others, both ischemic deterioration and elevated thromboxane release occurred after operation. These patients had preoperative values similar to the values in those without ischemic symptoms. The observations suggest that increased platelet aggregability and thromboxane release are associated with delayed cerebral ischemia both before and after surgery.