A 2-year-old boy with hemimegalencephaly and Hirschsprung's disease is reported. The unique association of these two entities is considered to he the presence of a common insult or insults that affect the innervation of the bowel and the formation of the cerebral cortex. Short-segment subtype of Hirschsprung's disease may suggest that this effect occurred between the eighth and twelfth weeks of gestation. Although there is a well-known coexistence of Hirschsprung's disease with the malformations that share a common neurocristopathic origin (abnormalities of neural crest cell growth, migration, or differentiation), a few extremely rare cases, as in this case, might reflect the coexistence of Hirschsprung's disease with a cerebral malformation (i.e., hemimegalencephaly) that is a nonneurocristopathic entity by itself. (C) 1998 by Elsevier Science Inc, All rights reserved.