Aspirin may reduce ischemic brain injury. The aim of this study was to explore the effect of aspirin on glutamate release after acute stroke. We studied 238 patients with a first episode of hemispheric ischemic stroke of less than 24 h duration. Early neurological deterioration was diagnosed when the Canadian Stroke Scale dropped 1 or more points between admission and 48 h. Glutamate was determined on cerebrospinal fluid (CSF) samples obtained at admission. Sixty-three patients were undergoing treatment with 75-500 mg/day of aspirin at the time of stroke onset. CSF glutamate concentrations were higher in the group of patients not taking aspirin (8.9+/-5.2 vs. 4.9+/-3.1 microM/l, P< 0.0001). Aspirin treatment at stroke onset had a 97% risk reduction of early neurological deterioration, and this effect remained unchanged after a further adjustment for glutamate concentrations. These findings suggest that low doses of aspirin may be useful in the management of patients with cerebral ischemia, not only for its antithrombotic properties, but also by direct neuroprotective effects.