Deletions of the 15q26 region encompassing the chromodomain helicase DNA binding domain 2 (CHD2) gene have been associated with intellectual disability, behavioral problems, and several types of epilepsy. Including the cases mentioned in ECARUCA (European cytogeneticists association register of unbalanced chromosome aberrations) and DECIPHER (database of genomic variation and phenotype in humans using ensembl resources), so far, a total of 13 intellectually disabled patients with a genetically proven deletion of the CHD2 gene are described, of whom eleven had a history of severe forms of epilepsy starting from a young age. In this article, a moderately intellectually disabled 15-year-old male with a 15q26.1-q26.2 interstitial deletion is reported, who was referred for analysis of two recent short-lasting psychotic episodes that were nonresponsive to antipsychotic treatment and recurrent disinhibited behaviors since early infancy. Careful interdisciplinary assessment revealed that the psychotic phenomena originated from a previously unrecognized absence epilepsy. Treatment with valproic acid was started which resulted in full remission of psychotic symptoms, and consequently, substantial improvement of behavior. It was concluded that in case of (rare) developmental disorders with genetically proven etiology, a detailed inventory of anamnestic data and description of symptomatology over time may elucidate epilepsy-related psychopathology for which a specific treatment regimen is needed.