Phelan–McDermid or 22q13.3 deletion syndrome is characterized by global intellectual disability, childhood hypotonia, severely delayed or absent speech, features of autism spectrum disorder, without any major dysmorphisms or somatic anomalies. It is typically diagnosed before adolescence and data about adult patients are virtually absent. The expression of its phenotypical characteristics appears to be linearly related to the deletion size. Here, an intellectually disabled geriatric female patient is described with a long history of challenging behaviors in whom Phelan–McDermid syndrome was demonstrated. Detailed analysis of the patient's history and functioning resulted in a psychiatric diagnosis of atypical bipolar disorder and her behavior significantly improved upon maintenance treatment with a mood stabilizing agent. The present article confirms recent findings that atypical bipolar disorder may be part of the psychopathological phenotype of Phelan–McDermid syndrome, reason why careful etiological search is warranted, also in the geriatric population. © 2012 Wiley Periodicals, Inc.