We report three siblings born with severe neonatal encephalopathy, manifested clinically by microcephaly, myoclonus, and muscular hypertonus. Karyotypic analyses and all biochemical investigations were unrevealing. All three patients died during infancy. Postmortem examination of the brain in one child disclosed severe neuronal loss in the inferior olives and the pontine nuclei. There was also severe hypoplasia of the cerebellum and micrencephaly. There was diffuse gliosis of the white matter in all areas of the brain. We believe this may represent a previously undescribed form of familial infantile encephalopathy with olivopontocerebellar hypoplasia.