Macrocerebellum, a neuroradiological and clinical entity of unknown etiology characterized by an isolated, disproportionately large cerebellum, has to date been reported in only a few cases. It has been suggested that the condition could represent a marker for disturbed cerebral development, however, longitudinal reports are lacking. We describe a 19-month-old patient with agenesis of the corpus callosum, who developed enlargement of the cerebellum without clinical signs of cerebellar impairment, a picture that has not been previously described.