The 17q21.31 microdeletion syndrome with its characteristic features including developmental delay, moderate intellectual disability, facial dysmorphisms, and anomalies of the brain and multiple organ systems was recently described. As to its behavioral profile, scarce data from clinical observations have suggested a remarkably amiable, friendly disposition, to some extent comparable to that observed in Angelman and Williams syndromes. The present study focuses on the various aspects of neurocognitive functioning, particularly social cognition, in patients with 17q21.31 microdeletion syndrome. Neuropsychological assessment was performed in three out of the four known Dutch patients with a genetically proven 17q21.31 microdeletion syndrome. Apart from developmental age, cognition and social-emotional functioning was extensively assessed. In addition, data of three intellectually disabled physically healthy reference subjects, recruited from a small outpatient sample, were included. The general cognitive profile of all subjects was in accordance with their lowered intellectual capacities, albeit that in patients with the 17q21.31 microdeletion, a relatively strong memory for social-contextual information was found. Basic emotion perception was intact, but patients with the 17q21.31 microdeletion syndrome showed less social fear and more approaching behavior. Interestingly, alexithymic traits, that is marked difficulties in the recognition and expression of emotions, were more prevalent in reference subjects. Despite the methodological limitations characteristic for research in people with intellectual disabilities, with a neuropsychological assessment strategy, in three patients with 17q21.31 microdeletion syndrome, preliminary evidence for hypersocial behavior with a high level of frustration tolerance was found that may be implicated in its behavioral phenotype. © 2012 Wiley Periodicals, Inc.