Background: The "Stay Well and Healthy!" Health Risk Appraisal (SWH-HRA) tool was developed and piloted in an in-home preventive healthcare program for persons ageing with intellectual and developmental disabilities (Aronow & Hahn 2005; Hahn & Aronow 2005). This paper presents the results of reliability and validity assessment of the SWH-HRA tool based on pilot data for 201 participants with intellectual and/or developmental disabilities (IDD) who were administered the SWH-HRA, a subsample of 36 who were administered the SWH-HRA twice within 100 days, and a subsample of 69 participants who received an advanced practice nurse (APN) (nurse practitioner) intervention. Materials and Methods: The reliability of the SWH-HRA tool was assessed using descriptive statistics, scale item analyses and correlation analyses. Descriptive statistics were run on all health risk measures of the SWH-HRA, and the results were examined for face validity. Cronbach's alpha statistics were calculated for all multi-item scales. Factor analyses were used to evaluate the factor structure in three scales. Correlation statistics were used for test-retest reliability and for validity assessment comparing selected SWH-HRA scales with comparable measures used by the APN. Criterion validity was assessed by evaluating the relationship of the risk items or scales to use of emergency room and hospital services and to participation. Results: The findings of the psychometric analysis of the SWH-HRA tool, which was developed for assessing health and risks of persons with intellectual and developmental disabilities, support the tool's overall reliability and validity. Conclusions: The findings add support for continued use and testing of the SWH-HRA in randomized clinical control trials comparing SWH-HRA use to usual care with persons with IDD. Further psychometric testing is indicated with persons with other types of disabilities.