A case of X-linked hydrocephaly is presented. Early second trimester evaluation of the size of the lateral cerebral ventricles resulted in one male fetus continuing further normal development and one male fetus being aborted because of progressive hydrocephaly. The affected fetus was characterized by cerebral ventricular dilation without aqueductal stenosis. This case history shows the feasibility of early monitoring of pregnancies at risk of X-linked hydrocephaly. In some cases, ventricular enlargement rather than aqueductal stenosis may be the primary lesion.