The clinical symptoms associated with chromosome 15q duplication syndrome 
manifest through a heterogeneous group of symptoms characterised by hypotonia, 
delay in motor skills and language development, cognitive and learning 
disabilities, autism spectrum disorder and refractory epilepsy. The late 
development of Lennox-Gastaut syndrome in patients with 15q11q13 duplication is 
a possibility that physicians should be aware of. We report the case of a 
27-year-old man with a neurodevelopmental syndrome due to a 15q duplication, 
with intellectual disability, psychiatric disturbances, and an epileptic 
phenotype diagnosed as late-onset Lennox-Gastaut syndrome.