Angelman syndrome (AS) is a neurodevelopmental disorder characterized by severe mental retardation, ataxia, and a happy/sociable disposition. Maternally, but not paternally, derived defects, such as duplications, within the AS critical region result in autistic symptomatology, suggesting that the UBE3A gene might be implicated in the causation of autism. This study examined the prevalence of autism in AS in 19 children representing three known molecular classes of AS. Children were studied over the course of 1 year. Forty-two percent of this population, eight of 19 children, met criteria for autism according to the Autism Diagnostic Observation Schedule (ADOS). Parents of children who were diagnosed with autism according to Diagnostic and Statistical Manual of Mental Disorders (DSM)-IV criteria as well as the ADOS - Generic, Module 1 (ADOS-G) were administered the Autism Diagnostic Interview - Revised (ADI-R). Data from the ADI-R were convergent with data from the ADOS-G in all cases. Children with comorbid autism and AS scored lower on measures of language, adaptive behavior, and cognition, and demonstrated a slower rate of improvement over the course of the study. Furthermore, they demonstrated deficits in communication and socialization that mirror those observed in children with idiopathic autism. The study highlights the phenotypic overlap between autism and AS and increases the probability that dysregulation of UBE3A may play a role in the causation of autism.