AbstractBetween February 1970 and December 1971, 200 premature infants with birth weights of up to 2500 g were examined echoencephalographically within 48 hours of birth. The third ventricle and the brain‐mantle‐index (BMI) were determined. It was statistically proved that with increasing weight, advancing gestational age and growth of head circumference, the width of the third ventricle diminishes. The breadth of the third ventricle in premature infants and so‐called “small‐for‐date‐infants” was found to be essentially the same. An enlarged width of the third ventricle in cases of low birth weight and low gestational age, could possibly represent a physiological phase. BMI and third ventricle correlated closely. Conclusion: The more fully developed the premature infant, the smaller the width of the ven