Children (156) born with myelomeningocele were studied over a 3-10 yr follow-up period. The appearance, type and rate of development of hydrocephalus were noted and correlated with the findings on neonatal and follow-up physical examination. Of the children 80% developed overt hydrocephalus; 73.4% of these cases were secondary to aqueductal stenosis. Only 15.3% of the cases showed physical evidence of hydrocephalus at birth. Another 64.8% of the children developed clinical signs of hydrocephalus during the follow-up period, most during the 1st few weeks but some as late as the 3rd yr of life. The neonatal examination was unable to predict the subsequent appearance of hydrocephalus or its rate of development, and even serial determinations of head circumference were often of limited value. The early diagnosis of hydrocephalus in children with myelomeningocele cannot reliably be made on physical examination alone and other means of assessing ventricular size should be employed.