The aim of our study is to offer new objective diagnostic and prognostic criteria of neurological disturbances in children with the birth weight less than 1500g during the first year of life. 102 extremely premature infants with hypoxic-ischaemic encephalopathy were observed. Them were performed the clinical, ultrasound, electroencephalographic (EEG) and biochemical studies during the first year of life. The diagnostic value of the glial protein S100 blood index in the early neonatal period were determined. There were found that the sigma rhythm quantitative characteristics of EEG studies in the corrected age of 1 month have prognostic significance for neurological outcome.