The Dandy-Walker syndrome (DWS) is a rare congenital posterior fossa malformation characterized by aplasia or hypoplasia of the cerebellar vermis, cystic dilatation of the fourth ventricle, and enlargement of the posterior fossa. We present a 52-year-old Caucasian man diagnosed with gastrointestinal stromal tumor and submitted to 18F-FDG PET/CT as a staging procedure. The patient was previously diagnosed with DWS in brain CT scan. PET/CT images revealed an ametabolic large cyst in the posterior fossa and hypoplasia of cerebellar vermis. The case is presented with the aim to show the appearance of this syndrome on PET/CT study.