This report describes an exceptional case of X (49, XXXXX) pentasomy in a girl aged three years and five months. She was admitted for recurrent seizures revealing epilepsy. She has growth failure and psychomotor retardation with a deformed face. The malformative assessment did not show any malformation apart from cerebral leukodystrophy. Pentasomy X is a very rare abnormality of the sex chromosomes. It only affects females, in whom three additional X chromosomes are added to the two X normally present. The pathogenesis of pentasomy X is not exactly clear, but it is probably caused by successive maternal nondisjunctions. Epilepsy and cerebral leukodystrophy are a new mode of revelation of this syndrome, never described in the literature.