Our goal was to determine the neuron-specific enolase (NSE) concentration in cerebrospinal fluid (CSF) and plasma in patients with the acute brain infarction (BI) and analyze the correlation between the measured NSE concentration and infarct volume and the degree of neurological and functional deficit. The study included 55 patients aged 56-68 with BI in the acute phase. The control group consisted of 16 patients subjected to diagnostic radiculography. The results showed a significant increase of NSE concentration within the first seven days in patients compared to the controls (2.838 +/- 0.504 ng/ml CSF and 4.479 +/- 0.893 ng/ml plasma). A significant correlation was found between NSE concentration and infarction volume and the degree of neurological and functional deficit both in the CSF (r = 0.828, r = 0.735, r = 0.796; p < 0.001) and in plasma (r = 0.810, r = 0.681, r = 0.783; p < 0.001). The results suggest that an early determination of this marker in CSF and plasma in patients with BI could be a valuable diagnostic factor.