A family is described with congenital hydrocephalus occurring in two pregnancies following the birth of a child with a neural tube defect (NTD). Prenatal diagnosis of hydrocephalus at mid-gestation was achieved by ultrasonography. The increased frequency of hydrocephalus among sibs of probands with a NTD and vice versa suggests that, following the birth of a child with either malformations, subsequent pregnancies should be monitored at mid-gestation by amniotic fluid AFP and serial ultrasound examination.