Lateral cerebral ventricular volume in 36 preterm infants with or without an intraventricular hemorrhage, and with or without posthemorrhagic hydrocephalus, was measured longitudinally and compared with the ventricular index measurements of the same ventricles. A poor correlation was found (r2 = 0.67). To determine a reason for this poor relationship, we analyzed the volumes of the regions of the ventricles by a segmental volume analysis. The occipital region of the lateral cerebral ventricle enlarged at a much faster rate (1.904 +/- 0.477 ml/day) than either the anterior region (0.546 +/- 0.253 ml/day; p less than 0.01) or the middle region (-0.209 +/- 0.334 ml/day; p less than 0.01) in infants with posthemorrhagic hydrocephalus. The rate of growth of the middle region of the lateral cerebral ventricles was the same for all infants. Linear indexes, such as the ventricular index and the lateral ventricular ratio, do not allow for accurate serial estimates of ventricular size in posthemorrhagic hydrocephalus because of asymmetric growth of the lateral cerebral ventricle. We conclude that sequential volume measurements are more useful than ventricular index measurements to follow ventricular size sequentially in infants with posthemorrhagic hydrocephalus.