This follow-up study was designed to evaluate whether the use of aspirin either before or after aneurysm rupture affects the occurrence of delayed cerebral ischemia. Aspirin inhibits platelet function and thromboxane production and has been shown to reduce the risk of various cardiovascular and cerebrovascular ischemic diseases. Following admission, the patients in this study was interviewed regarding their use of aspirin and other medicines prior to and after hemorrhage, and their urine was screened qualitatively for salicylates. Patient outcome and the occurrence of hypodense lesions consistent with cerebral infarction on follow-up computerized tomography (CT) were studied prospectively up to 1 year after hemorrhage. Of 291 patients, 31 (11%) died because of the initial hemorrhage and 18 (6%) died due to rebleeding within 4 days after hemorrhage. Of the remaining 242 patients, 90 (37%) had delayed cerebral ischemia, which caused a permanent neurological deficit or death in 54 patients (22%). Of 195 patients undergoing follow-up CT, 85 (44%) had cerebral infarction that was not seen on the CT scan obtained on admission. Those who had salicylates in the urine on admission had a relative risk of 0.40 (95% confidence interval (CI), 0.15 to 1.10) of delayed ischemia with fixed deficit and a risk of 0.40 (95% CI, 0.18 to 0.93) of cerebral infarction compared with patients who did not have salicylates in their urine. This reduced risk of ischemic complications with aspirin use was restricted to those patients who used aspirin before hemorrhage, when the risk of ischemia was 0.21 (95% CI, 0.03 to 1.63) and the risk of infarct was 0.18 (95% CI, 0.04 to 0.84) compared with those who had not used aspirin. The reduced risk of cerebral infarction remained significant after adjustment for several potential confounding factors (adjusted risk 0.19; 95% CI, 0.04 to 0.89). These observations suggest that platelet function at the time of subarachnoid hemorrhage may be associated with delayed cerebral ischemia after aneurysm rupture.