A 48-year-old woman with late infantile onset mental retardation developed megacolon. Although the patient had no typical clinical features of Hirschsprung disease-mental retardation syndrome, a new 3-base pair deletion, eliminating an Asn, was identified in the responsible gene ZFHX1B. This suggests that screening for ZFHX1B mutations is warranted even in the absence of typical clinical features of the syndrome.