Hydrocephalus occurs in 69% to 92% of the meningomyelocele population, but rarely becomes manifest until after the meningomyelocele is closed. Ventriculomegaly is common at birth, even in neonates without overt hydrocephalus. Thus, palpating the anterior fontanel and cranial sutures and measuring the head circumference may be misleading. We report a means of identifying spina bifida neonates who will subsequently develop hydrocephalus. Dubowitz gestational age, birth weight, birth head circumference, head circumference percentile, and the lateral ventricular ratio (LVR) from ultrasonograms were analyzed for each of 25 neonates with meningomyelocele. The mean head circumference percentile was 47.7 +/- 7.7 SE (range: less than 5 to greater than 95). Ventriculomegaly, however, defined by an LVR of greater than 0.32, was present in all but 2 of the neonates. Pearson's correlation test showed that only one-third of the elevated LVRs could be explained by the head circumference. Later ventricular shunting was eventually required in all but 3 infants. We concluded that (1) clinical examination of the myelodysplastic neonate usually does not reveal evidence of hydrocephalus and (2) ventriculomegaly on ultrasonography predicts the later development of hydrocephalus following meningomyelocele closure.