A growing skull fracture or leptomeningeal cyst most commonly occurs in children under the age of 3 years, and is extremely rare in adults. The reason for a growing skull fracture is usually a dural tear in association with the fracture. This paper presents an 18-year-old mentally retarded patient with cerebral hemiatrophy (Dyke-Davidoff-Masson syndrome) associated with a growing skull fracture in the ipsilateral hemicranium, in whom not only a dural tear but also the ipsilaterally displaced and dilated lateral ventricle due to the original disease apparently contributed to the development of growing skull fracture.