Given the growing importance of support needs assessment in the field of intellectual disability, it is imperative to develop assessments of support needs whose scores and inferences demonstrate reliability and validity. The purpose of this study was to examine the criterion validity of scores on the "Supports Intensity Scale-Children's Version" (SIS-C) by identifying the relation of SIS-C scores to those on the "Supports Intensity Scale-Adult Version" (SIS-A) for youth on the boundary of appropriateness of the two assessments (ages 16-21). Using data from 142 youth who both completed the SIS-A and SIS-C, we found that parallel support need constructs on the two versions of the SIS have strong associations. In addition, there were similar relations between personal competency (i.e., intelligence and adaptive behavior) and support needs measured by the SIS-A and SIS-C. Implications for future research and practice are discussed.