Inappropriate sexual behaviours observed in autism spectrum disorder (ASD) are difficult for both the family and the child, and they can significantly deteriorate social adaptation and functionality. Therefore, these behaviours need to be addressed and treated appropriately. Conventional treatments may be inadequate in a number of cases. In this case report, the treatment of inappropriate sexual behaviours using gonadotropin-releasing hormone (GnRH), despite the medical and behavioural treatment of a 13-year-old boy diagnosed with ASD with mental retardation comorbidity, is described. The use of hormonal therapy in inappropriate sexual behaviours is discussed.