The 4 Mb 15q11–q13 region is prone to structural rearrangements. Deletions have been identified among the leading causes for genetic diseases such as the Prader–Willi and Angelman syndromes, while duplications, occurring preferentially on the maternal chromosome, produce a typical phenotype that includes mental retardation, language delay, seizures and autism. Although a number of such patients have been reported, however, there is a paucity of information about their clinical outcomes in adult age. We report on a 33-year-old female with a microduplication of 15q11–q13 detected by array-CGH analysis, with particular reference to the epilepsy phenotype, characterized as a late-onset Lennox–Gastaut syndrome. © 2009 Wiley-Liss, Inc.