Infantile olivopontocerebellar atrophy (OPCA) is a rare congenital disorder likely due to an intrauterine neurodegenerative condition. Characteristic presentations are failure to thrive, cerebellar ataxia, respiratory insufficiency, and hypotonia or hypertonia. A few cases with severe manifestations (eg, the Pena-Shokeir phenotype) presenting in the neonatal period have also been reported. We present a case of infantile OPCA with the Pena-Shokeir II phenotype and severe atrophy of the cerebellum and cerebral hemispheres. Comparison of prenatal sonographic findings of the fetal brain at 30 weeks' menstrual age and CT findings during the neonatal period indicated prenatal onset of the neurodegenerative process, which progressed rapidly during the last trimester.