The study includes a continuous and unselected group of 125 premature infants whose birth weight was below 2500 gr. The echo-encephalogram was done at the time when the infants had reached a weight of 2500 gr. The width of the third ventricle and the brain mantle index (relation between the distances of midline echo-end echo and echo of the lateral wall of the temporal horn-end echo) were measured. The third ventricle showed a normal width of 2--3 mm which was independant from birth weight and gestational age. There were only two infants with higher values, one of them had different symptoms of brain damage. The calculation of the brain mantle index showed unsatisfactory results. The values varied between 2,35 and 4,91. They were markedly beyond the normal value of 2,4 of adults and bigger children. Technical difficulties in temporal horn and inaccuracy in reading the results on the screen as a possible cause of these discrepancies are disussed.