Rhombencephalosynapsis is a rare congenital abnormality characterized by noncleavage of the cerebellar hemispheres and dentate nuclei with agenesis or severe hypoplasia of the cerebellar vermis. We report an 8-month-old boy with hypotonia, mental and motor retardation, and capillary hemangiomas on his pretibial region. Cranial magnetic resonance imaging revealed vermian hypoplasia with fused cerebellar hemispheres called rhombencephalosynapsis. Other cerebellar and cerebral structures were normal. With this constellation of findings, we attempt to identify the pathogenesis of this posterior fossa malformation. We also discuss cerebellar embryogenesis to differentiate this condition from other posterior fossa malformations such as Dandy-Walker malformation and Joubert's syndrome.