Recent clinical and experimental studies suggest that ischemic strokes may play an important role in the pathogenesis of Alzheimer's disease (AD). Beta amyloid (Abeta), a major component of senile plaque in AD, is known to be derived from ischemic brain or activated platelets. We prospectively enrolled 62 patients with acute ischemic stroke and 27 age-matched controls. The serum Abeta and P-selectin levels were determined using the Sandwich-ELISA. We divided ischemic strokes into subgroups according to the clinical syndrome, pathogenesis, and infarct size, and compared the Abeta level between each subgroup. The Abeta1-40 level was markedly elevated in ischemic stroke patients, as compared to controls (140.2 +/- 54.0 vs 88.44 +/- 34.96 pg/ml, p<0.001). Cardioembolic and larger artery atherosclerotic infarcts had higher Abeta1-40 level than small vessel disease (p = 0.001). Both infarct size and the initial NIHSS score had significantly positive correlations with the serum level of Abeta1-40 (r = 0.539, p<0.001 and r = 0.425, p = 0.001, respectively). However, the P-selectin level was not significantly correlated with serum Abeta1-40. Our data suggest that elevated circulating Abeta1-40 in ischemic stroke patients may be derived from brain as a consequence of ischemic insults.