Sex chromosome anomalies have been previously associated with several brain malformations including posterior fossa anomalies, such as cerebellar dysplasia or hypoplasia, cerebellar cysts, vermis dysgenesis or hypoplasia, and mega cistern magna. XYY syndrome is a sex chromosome aneuploidy characterized by an extra copy of the Y chromosome. Although it has been proposed that the presence of such extra chromosome may have an adverse effect on brain development, to date few reports on brain abnormalities in patients with XYY syndrome have been published. In a male child with 47, XYY karyotype we describe a particular brain malformation which consisted of enlarged posterior fossa and hypoplasia of posterior and inferior regions of left cerebellar hemisphere and vermis. In addition we revised other sex chromosome anomalies which have been previously associated with posterior fossa malformations in humans. Conclusion: Our finding suggests that having an extra Y chromosome may affect brain development. Brain radiological imaging in patients with XYY syndrome would be useful to determine whether such brain abnormalities are an incidental finding or part of the spectrum of XYY syndrome. A deeper investigation of the extra chromosome effects may help to better comprehend the pathophysiology of functional disorders in affected individuals.