The outcome of four successive pregnancies in a woman heterozygous for X linked hydrocephalus is described. The last two were scanned by ultrasound. In one, a good prognosis was given; the fetus was male but there was no evidence of dilated cerebral ventricles. In the other, hydrocephalus was diagnosed. The absence of aqueductal stenosis in this case supports the hypothesis that in this X linked condition communicating hydrocephalus is the primary defect and aqueductal stenosis is secondary.