Hemimegalencephaly (HME) is an uncommon sporadic nonfamilial congenital dysplastic abnormality of the central nervous system, characterized by enlargment of one cerebral hemisphere, with cranial asymmetry, hemiparesis, epilepsy, and mental retardation. It can occur in isolation or associated with various anomalies, namely skin disorders. The main neuropathologic findings are hemispheric gigantism, macro- and/or micropolygyria, cortical thickening with lack of lamination, blurred boundaries of the gray and white matter, and large ortho- and heterotopic neural cells. The results obtained by morphological investigations carried out on six patients with HME, compared with the findings recorded in similar studies performed on one patient with tuberous sclerosis (TS) and another with pachygyria, allow the authors to (a) confirm the dysplastic nature of HME and its autonomy from TS; (b) demonstrate that ortho- and heterotopic neuronal cells do not differentiate completely during proliferation and migration from the germinal matrix; (c) document, by means of flow cytometric study, a normal euploid DNA content in the enlarged hemisphere, consequently ruling out heteroploidy as a cause of both cell “hypertrophy” and enlargement of the malformed cerebral hemisphere.