Comparing echoencephalographic examinations in 71 premature infants and 200 children from birth to the 18th month of living. Determination of the 3rd ventricle and of the brain-mantle-index (BMI). In both groups an increase of the extent of the 3rd ventricle was found, whereby the dates for the premature infants with a birth weight of 1500—2500 g from the 6th month of living showed an about 1,6—1,9 mm larger 3rd ventricle. Premature infants under 1500 g showed clearly wider 3rd ventricles up to extents of 7 mm with always pathological BMI. Small for date infants showed a tendency to higher dates, too. The larger extent of the 3rd ventricle of premature infants with small birthweight respectively low gestational age may likely not looked at as a physiological passing stage. One should refrain from standardisation for the extent of the 3rd ventricle and the ventricular system at present time.