Proposed DSM-5 revisions to the diagnosis of autism spectrum disorder (ASD) include a “severity” marker based on degree of impairment. Although qualitative differences between support levels are described, quantitative methods or practice recommendations for differentiating between levels remain undetermined. This leaves the field vulnerable to potential discrepancies between severity categorizations that may have inadvertent service implications. We examined overlap between mild, moderate, and severe impairment classifications based on autism symptoms, cognitive skills, and adaptive functioning in 726 participants (15 months—17 years) with ASD. Participants with mild, moderate, and severe autism symptoms demonstrated varying levels of adaptive and cognitive impairment. These discrepancies highlight the need for a clearly elucidated method of classifying level of support in ASD diagnosis.