Objective: This study evaluated the potential impact of proposed "DSM-5" diagnostic criteria for autism spectrum disorder (ASD). Method: The study focused on a sample of 933 participants evaluated during the "DSM-IV" field trial; 657 carried a clinical diagnosis of an ASD, and 276 were diagnosed with a non-autistic disorder. Sensitivity and specificity for proposed "DSM-5" diagnostic criteria were evaluated using field trial symptom checklists as follows: individual field trial checklist items (e.g., nonverbal communication); checklist items grouped together as described by a single "DSM-5" symptom (e.g., nonverbal and verbal communication); individual "DSM-5" criterion (e.g., social-communicative impairment); and overall diagnostic criteria. Results: When applying proposed "DSM-5" diagnostic criteria for ASD, 60.6% (95% confidence interval: 57%-64%) of cases with a clinical diagnosis of an ASD met revised "DSM-5" diagnostic criteria for ASD. Overall specificity was high, with 94.9% (95% confidence interval: 92%-97%) of individuals accurately excluded from the spectrum. Sensitivity varied by diagnostic subgroup (autistic disorder = 0.76; Asperger's disorder = 0.25; pervasive developmental disorder--not otherwise specified = 0.28) and cognitive ability (IQ less than 70 = 0.70; IQ [greater than or equal to] 70 = 0.46). Conclusions: Proposed "DSM-5" criteria could substantially alter the composition of the autism spectrum. Revised criteria improve specificity but exclude a substantial portion of cognitively able individuals and those with ASDs other than autistic disorder. A more stringent diagnostic rubric holds significant public health ramifications regarding service eligibility and compatibility of historical and future research. (Contains 4 tables.)