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Klinefelter’s syndrome (KS) with the classic 47, XXY karyotype is the most frequent chromosomal aneuploidy, with a prevalence of 1 in 700 men and is the most common genetic form of male hypogonadism. Men with KS have an increased prevalence of psychiatric disturbances, ranging from attention deficit disorder in childhood to schizophrenia and severe affective disorders during adulthood. 47, an XXY aneuploidy is found in about 0.8–1% of men hospitalized for schizophrenia, representing a four-folds to five-folds excess over the incidence at birth of Klinefelter Syndrome. We want to present a case report of KS who concurrently suffer psychosis. A 23 year-old, single man was admitted to the outpatient clinic of psychiatry because of acute stress disorder symptoms such as terror, insomnia, social detachment, irritability, 3 days after the Reyhanlı terrorattack. He was diagnosed to have mild Mental Retardation due to behavioral problems such as aggression, irritability in childhood and also diagnosed Klinefelter’s Syndrom (47, XXY) with cytogenetic study and tricuspid regurgitation with detailed cardiological examination when he was 13. He was diagnosed to have psychosis because of persecutory delusions when he was 18 and Olanzapine treatment was started. His past psychiatric history was significant for a diagnosis of paranoid schizophrenia with several psychiatric hospitalization and olanzapine treatment increased to about 30 mg/day. Currently, he is under control with olanzapine 10 mg/day and there is no psychotic symptom. His affect was depressed and he also appeared anxious and irritable. Physical examination indicated that he has long arms and legs relatively. All other investigations, including blood count, renal and hepatic function, electroencephalogram, and serum testosterone levels were all within normal limits. KS is associated with multiple psychiatric comorbidities. Bruining et al. found that language disorder has a comorbidity of 65% as the most prevalent disorder followed by attention deficit hyperactivity disorders (63%) and autism spectrum disorder (27%) in KS. Behavioral impairment was most evident among cases classified as autism spectrum disorder and psychotic disorder (12%). A survey of hospital admissions and discharge diagnoses among individuals with XXY in Denmark and a randomly selected age-matched control group found that individuals with XXY had an increased relative risk of being hospitalized for psychiatric disorders, particularly for psychoses. The psychological symptoms were very similar to those cases mentioned in other reports of KS, associated with schizophrenia-like symptoms. Some reports about the relationships between sex hormones and schizophrenia including other psychotic disorders suggest that the X-chromosome plays an important part in the mechanism of psychosocial symptoms and in the prognosis in KS.