Infants with periventricular-intraventricular hemorrhage (PV-IVH) were followed up with weekly ultrasound sector scans to define the natural history of late ventricular dilation (ie, dilation in excess of that seen at the time of diagnosis of PV-IVH). Infants fell into two groups: (1) posthemorrhagic hydrocephalus (PHH), dilation that produced an increase in occipitofrontal circumference greater than 2 cm per week and/or clinical symptoms of increased intracranial pressure; and (2) ventriculomegaly (VM), dilation that stabilized or reversed without producing these effects. The former was directly related to the severity of hemorrhage. The diameter of the lateral ventricle was significantly greater in PHH. In 26 of 48 infants at risk, late dilation developed: 14 had VM and 12 had PHH. Thus, late ventricular dilation stabilized or resolved spontaneously in 54%. Only three infants eventually required a ventriculoperitoneal shunt. Clinical changes in addition to ventricular size should be used in assessing the need for treatment of ventricular dilation after PV-IVH.