Phelan-McDermid syndrome (PMS) is a multiple congenital anomalies and intellectual disabilities syndrome associated with a deletion of chromosome 22 terminal band 13.3. The deletion is associated with severe intellectual disabilities, absent or delayed speech, behavior problems, and autism. The objective of this study was to provide a detailed assessment and analysis of problematic behaviors in this population. Semistructured parent interviews, checklists, and record reviews were conducted for 35 families with children with PMS. Parents participated in completing the Vineland Adaptive Behavior Scales-II, Reiss Scales for Children's Dual Diagnosis (Reiss), and the Parent Form of the Children's Interview for Psychiatric Symptoms (P-ChIPS). Children with PMS showed adaptive behaviors more than 3 standard deviations below the mean. Maladaptive behaviors were present in nearly all children. Moreover, the P-ChIPS and interview results showed evidence of unstable mood, depressive symptoms, and overactivity. The Reiss showed high scores for psychosis, autism, depression, and attention deficit. Children with PMS have high levels of maladaptive behaviors as well as evidence of mood, attention, autistic, and psychotic issues reported by parents. Although PMS previously has been associated with autism, there are confounds between autism and mental issues in this rare population. Implications for understanding the nature of autism and mental health disorders in children are discussed. (Contains 5 tables and 2 figures.)