We report a case of isodicentric chromosome 15 (idic(15) chromosome), the presence of which resulted in uncontrolled seizures, including epileptic spasms, tonic seizures, and global developmental delay. A 10-month-old female infant was referred to our pediatric neurology clinic because of uncontrolled seizures and global developmental delay. She had generalized tonic-clonic seizures since 7 months of age. At referral, she could not control her head and presented with generalized hypotonia. Her brain magnetic resonance imaging scans and metabolic evaluation results were normal. Routine karyotyping indicated the presence of a supernumerary marker chromosome of unknown origin (47, XX +mar). An array-comparative genomic hybridization (CGH) analysis revealed amplification from 15q11.1 to 15q13.1. Subsequent fluorescence in situ hybridization analysis confirmed a idic(15) chromosome. Array-CGH analysis has the advantage in determining the unknown origin of a supernumerary marker chromosome, and could be a useful method for the genetic diagnosis of epilepsy syndromes associated with various chromosomal aberrations.