Rhombencephalosynapsis is an unfrequent malformation of the posterior fossa essentially characterized by vermian agenesis or hypogenesis, fusion of the cerebellar hemispheres and fusion of the dentate nuclei. Supratentorial abnormalities are usually associated. No specific clinical syndrome can be described in relation with this disorder. We report a case diagnosed by MRI in a Living patient. (C) 2000 Elsevier Science Inc. All rights reserved.