Rhombencephalosynapsis (RS) is a rare cerebellar malformation characterized by vermian agenesis or hypogenesis, fusion of the hemispheres, and closely apposed or fused dentate nuclei. This report concerns a 29-year-old Caucasian female with a history of profound retardation and chronic hydrocephalus. Multiple shunts had been placed and there had been several admissions for shunt infections. Autopsy confirmed massive hydrocephalus and revealed marked aqueductal stenosis. The cerebellum was small with vermian aplasia, closely apposed dentate nuclei, fusion of the middle cerebellar peduncles, and a small fourth ventricle. Associated defects included fusion of the inferior colliculi and absence of dorsal olivary nuclei. This spectrum of anomalies is consistent with RS and could be explained by an embryological defect of dorsal pattering that affects the "isthmic organizer" at the mesencephalic-metencephalic border. Molecular analysis of dorsalizing genes, such as Lmx1a, which regulate early developmental events at the pontomesencephalic junction, may reveal a mutation or mutations unique to RS.