Our study reports two nontwin sisters with late-onset Lennox-Gastaut syndrome and chromosome 15q duplication, showing the evolution, symptoms, diagnosis, and treatment of these patients, with the aim of increasing knowledge about this extremely rare association. They had a variety of generalized seizures types, intellectual disability, electroencephalogram with generalized epileptiform discharges less than 3 Hz, dysmorphisms, and genetic studies with the presence of duplicated chromosome 15. Cases reported here may be related to chromosomal changes inherited from their asymptomatic mother.