Klinefelter syndrome is a chromosomal disorder in which one extra X chromosome is present (47,XXY). Several other numeric variants of this syndrome are described that comprise one or more additional sex chromosomes such as 48,XXXY, 48,XXYY and 49,XXXXY. These rare conditions are often associated with increased risk for congenital malformations, additional medical problems, and a more complex psychological phenotype. Since 1963, apart from two infants, only four adult patients with a XXXYY pentasomy have been published as case report. The present paper critically reviews the existing literature and provides detailed assessments of a 25-year-old male with intellectual disability and autism. For the first time, this very rare pentasomy is now recorded using all information about developmental history as well as findings from genetic, somatic, endocrinological and neuropsychological examination. It is concluded that children born with abnormalities of the external genitalia should always be evaluated for genetic abnormalities in order to avoid unwanted delay of appropriately designed multidisciplinary medical and psychological treatment.