The American Psychiatric Association is making changes in the autism spectrum disorder (ASD) criteria for the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). In order to examine potential effects of the changing of the criteria, 330 adults with intellectual disability (ID) from two developmental centers were examined. However, due to the fact that the DSM-IV-TR/ICD-10 Checklist does not contain one of the restricted behavior items listed in the current proposed DSM-5 criteria, 41 participants were eliminated from the study. An additional 62 individuals were randomly removed from the study so that no one group was 1.5 times larger than any other group. This left a total of 227 individuals. These individuals were divided into three groups: those who met criteria for an ASD according to only DSM-IV-TR criteria, those who met criteria according to the proposed DSM-5 criteria, and controls with ID not meeting ASD criteria according to either diagnostic system. After statistical analysis, individuals in the DSM-5 group evinced significantly greater overall ASD core symptoms than those in the DSM-IV-TR group or controls. In addition, those in the DSM-IV-TR group exhibited significantly greater overall ASD core symptoms than those in the control group. Furthermore, we found that the percentage of adults diagnosed with ASD declined by 36.53% when using DSM-5 as compared to DSM-IV-TR criteria. Implications of these findings are discussed.