To establish new cross-sectional reference values for the size of the lateral ventricles in a large cohort of neonates between 24 and 42 weeks' gestational age (GA) as well as longitudinal reference values for the follow-up of very preterm infants born at less than 30 weeks' gestation.Institutional review board approval and parental written informed consent were obtained for this prospective cohort study of 625 neonates (58% male patients) with a median GA of 33.4 weeks (range, 24.7-42.6 weeks). All infants underwent cranial ultrasonography (US) within 4 days after birth to evaluate the size of the lateral ventricles. Scanning was repeated in 301 preterm and term neonates within the 1st week of life to assess the presence of ventricular reopening. Seventy-nine very preterm infants (GA, <30 weeks) were prospectively included for cranial US at term-equivalent age (TEA). US measurements were performed of the ventricular index (VI), anterior horn width (AHW), and thalamo-occipital distance (TOD). Statistical analysis was conducted by using a paired t test, multilevel analysis, and analysis of covariance.Cross-sectional reference values for the VI and TOD increased with maturity, whereas the AHW remained constant. Vaginal birth was independently associated with a slightly smaller AHW following birth and with an increase in AHW within the 1st week of life (P < .05). Preterm-born infants showed a larger ventricular size at TEA compared with term infants (P < .001).New cross-sectional and longitudinal reference curves were established for the size of the neonatal lateral ventricles, which may allow for early identification and quantification of ventriculomegaly due to either posthemorrhagic ventricular dilation or periventricular white matter loss.