We measured the size of the cerebral ventricles in 86 pre-term infants by ultrasonography every 2 to 3 months for one year. Their developmental quotients (DQ) were examined at the age of 1 to 2 years. Ventricular size in a control group of 50 normal full-term infants was also measured. The size of the cerebral ventricles in the full-term infants had increased by three months after birth, whereas that in the pre-term infants had increased not by chronological 3 months after birth, but by corrected 3 months of age. In the pre-term infants, the left ventricle was larger than the right at a few months after birth. However, there was no difference between left and right ventricular size in the normal full-term infants. Infants with a lower DQ had larger lateral ventricles. Although the sensitivity of ventricular enlargement as a predictor of mental development was 50%, the specificity was high.