Enlargement of the cerebral lateral ventricles is observed in several neuropsychiatric disorders with origins in early brain development. Lateral ventricle size is also predictive of poor neurodevelopmental outcome in premature infants. Three-dimensional (3-D) ultrasound (US) offers an improved methodology for the study of lateral ventricle volume in neonates and infants. To assess the validity of ventricle volume measures obtained with 3-D US, we compared the volumes obtained by 3-D US with magnetic resonance imaging (MRI) in seven infants. Ventricle volumes were determined using a computer-assisted image analysis program, IRIS. There was excellent correlation between ventricle volumes obtained with 3-D US and those obtained with MRI (intraclass correlation coefficient 0.92, F = 23.28, p = 0.00027), indicating that 3-D US provides valid measures of overall lateral ventricle volume compared to the "gold standard" of MRI. 3-D US can provide an economical and practical means of studying lateral ventricle volume in neonates, a neurostructural marker of abnormal brain development.