have two X chromosomes and two Y chromosomes in each of his body's cells. In a small percentage of cases, 48,XXYY syndrome results from nondisjunction of the sex chromosomes in a 46,XY embryo very soon after fertilization has occurred. This means that a normal sperm cell with one Y chromosome fertilized a normal egg cell with one X chromosome, but right after fertilization, nondisjunction of the sex chromosomes caused the embryo to gain two extra sex chromosomes, resulting in a 48,XXYY embryo. Diagnosis A karyotype is done to diagnose XXYY syndrome. Treatment consists of medications, behavioral therapies, and intensive community support. Management Patients generally need to be followed by an endocrinologist. If hypogonadism is present, testosterone treatment should be considered in all individuals regardless of cognitive abilities due to positive effects on bone health, muscle strength, fatigue, and endurance, with possible mental health/behavioral benefits, as well. Most children with XXYY have some developmental delays and learning disabilities. Therefore, these aspects should be checked and monitored: psychology (cognitive and social–emotional development), speech/language therapy, occupational therapy, and physical therapy. Consultation with a developmental pediatrician, psychiatrist, or neurologist to develop a treatment plan including therapies, behavioral interventions, educational supports, and psychotropic medications for behavioral and psychiatric symptoms should be arranged. Common diagnoses such as learning disability/ID, ADHD, autism spectrum disorders, mood disorders, tic disorders, and other mental health problems should be considered, screened for, and treated. Good responses to standard medication treatments for inattention, impulsivity, anxiety, and mood instability are seen in this group, and such treatment can positively impact academic progress, emotional wellbeing, and long-term outcome. Poor fine-motor coordination and the development of intention tremor can make handwriting slow and laborious, and occupational therapy and keyboarding should be introduced at an early age to facilitate schoolwork and self-help skills. Educational difficulties should be evaluated with a full psychological evaluation to identify discrepancies between verbal and performance skills and to identify individual academic needs. Expressive language skills are often affected throughout life, and speech therapy interventions targeting expressive language skills, dyspraxia, and language pragmatics may be needed into adulthood. Adaptive skills (life skills) are a significant area of weakness, necessitating community-based supports for almost all individuals in adulthood. Additional treatment recommendations based on the individual strengths and weaknesses in XXYY syndrome may be required. Prognosis Patients have an essentially normal life expectancy, but require regular medical follow-ups. History The first published report of