The Supports Intensity Scale--Adult Version (SIS-A) was designed to measure the intensity of support needed by adults with intellectual disability (ID). The purpose of this study was to investigate the validity of the SIS-A to measure the support needs of people with autism spectrum disorder (ASD). Using data from 19 jurisdictions in North America, findings from Welch's T-test showed that SIS-A scores for people with an ID diagnosis (no secondary diagnosis) and people with an ASD diagnosis (no secondary diagnosis) did not meaningfully differ. Moreover, SIS-A scores for people with an ID diagnosis (with and without a secondary diagnosis, except no cases with a secondary diagnosis of ASD) and people with an ASD diagnosis (with and without a secondary diagnosis, except no cases with a secondary diagnosis of ID) did not meaningfully differ. These findings suggest that the SIS-A is not operating differently when measuring the intensity of support needed by people with ID and ASD, and therefore it can be used with confidence with both populations. Implications of how information from standardized support needs assessments can be used to inform planning and decision-making are discussed.