We sought to assess the sonographic findings and postnatal outcome in fetuses with the prenatal diagnosis of asymmetric hydrocephalus. The sonograms from cases of asymmetric hydrocephalus diagnosed prenatally at our institution were reviewed. Postnatal outcome was obtained from maternal, neonatal, and pediatric records. Fourteen fetuses at 17.3 to 38.9 weeks' gestational age on prenatal sonography had a maximum ventricular measurement of 10.2 to 48.8 mm, with the degree of asymmetry ranging from 2.2 to 27.3 mm. Thirteen of 14 had a normal-sized contralateral ventricle. Other fetal anomalies identified at sonography included Dandy-Walker malformation, intraventricular hemorrhage, porencephalic cyst, hydronephrosis, pleural effusion, and mild dilatation of a renal pelvis. Eleven fetuses had follow-up prenatal sonography. Among these, ventricular dilatation resolved in 5, remained the same in 3, increased in 2, and decreased in 1. Postnatal outcome was normal in 6 cases (43%) and abnormal in 8 (57%), including 2 cases of in utero intracranial hemorrhage, 2 with congenital syndromes, 1 with an imperforate foramen of Monro, 1 with tuberous sclerosis, 1 with developmental delays, and 1 with cerebral palsy. Asymmetric unilateral hydrocephalus appears to represent an entity different from bilateral hydrocephalus in that there is less risk of perinatal death, there are fewer associated anomalies, and the overall prognosis is better. Outcome may be normal, but fetuses with increasing unilateral ventriculomegaly and cases associated with other brain abnormalities tend to have a poor neurologic outcome.