Mental illness, and its assessment and treatment, is a concern for caregivers and professionals working with people with intellectual disability (ID), as well as a cause of distress and disability for people with ID. Research in this area is hindered by difficulties associated with recognizing symptoms of mental illness and tracking their changes. The use of empirically derived and tested tools can improve clinical and research outcomes. This study compares proxy reports of mental health symptoms using the Psychiatric Assessment Schedules for Adults with Developmental Disabilities (PAS-ADD) Checklist to self-reported symptoms using an interview form of the Brief Symptom Inventory (BSI). While the two scales were statistically correlated, clinically they presented very differently. The PAS-ADD identified 23.6% of participants as scoring in the at-risk category of mental health symptoms, while self-rating of symptoms on the BSI resulted in 67.8% of participants scoring above the cutoff. Concerns regarding the use of the BSI for a wide range of people with ID are discussed, as well as the concerns of using proxy report for subjective topics such as mental health symptoms.