Lobar holoprosencephaly was identified with sonography in 12 fetuses between 21 and 35 weeks' gestation. A confident diagnosis was made in each case by a mid-coronal view of the brain demonstrating absence of the cavum septum pellucidum with fusion and squaring of the frontal horns. The only associated anomaly was Dandy-Walker malformation that occurred in three cases. All fetuses had mild to severe ventriculomegaly. Five pregnancies were terminated; there was one spontaneous abortion and six fetuses were delivered at term. A ventriculo-peritoneal shunt was implanted in four. Follow-up was available for five and revealed severe mental retardation in each case. Lobar holoprosencephaly is amenable to prenatal ultrasound diagnosis, although a differentiation with other cerebral malformations may be difficult at times. The outcome of affected infants remains uncertain, but neurological impairment occurs frequently.