The Lennox-Gastaut syndrome (LGS) is a severe epileptic disorder, characterized by developmental delay, typical electroencephalographic pattern and poor control of seizures. This epileptic syndrome generally presents in early childhood but exceptionally starts at the age of 5 years or later. We describe a patient who was classified as late-onset LGS, with evidence of mild hydrocephalus on CT. MRI added new diagnostic information by demonstrating laminar heterotopic grey matter.