We report the first case of prenatal diagnosis of fatal infantile olivopontocerebellar hypoplasia syndrome, OMIM 225753. Ultrasound findings noted at 28 weeks' gestation included polyhydramnios, a small stomach bubble, a small but morphologically normal cerebellum, dilatation of the fourth ventricle, and long periods of normal fetal movement punctuated by sudden bursts of violent seizure-like activity of the fetal extremities. At birth, the child was noted to be hypertonic, myoclonic, hyper-reflexic, demonstrated poor gastrointestinal motility, and had severe apneic episodes. Magnetic resonance imaging (MRI) demonstrated marked hypoplasia or atrophy of the cerebellum, pons and medulla, mild atrophy of the cerebral cortex, and mild ex vacuo venticulomegaly of the fourth, third and lateral ventricles. This child died from respiratory insufficiency at 14 days of age. The parents of this child had previously lost a child with similar clinical and anatomical findings. Prenatal sonographers should be aware of the existence of this rare syndrome and should look carefully at the size of the cerebellum in cases of polyhydramnios or when in utero 'seizure-like' activity is seen. The importance of establishing this diagnosis lies in the fact that it appears to have a very poor postnatal prognosis and is likely to be inherited as an autosomal recessive disease.