Thirty cases of fetal hydrocephalus were diagnosed at the perinatal ultrasound unit of Yale-New Haven Medical Center during a 41 month period. Prenatal ultrasound was found to be highly valuable in the detection of fetal hydrocephalus. However, the antenatal detection of associated anomalies was not as uniformly successful. The importance of a complete sonographic evaluation of intracranial and extracranial anatomy is stressed, as 10 patients with documented hydrocephalus (37%) had associated intracranial anomalies and 17 patients (63%) had associated extracranial anomalies. In this series, isolated hydrocephalus without associated anomalies occurred in only four patients (15%).