The assessment of functional health in chronic illnesses such as HIV/AIDS and rheumatoid arthritis is central to the measurement of health-related quality of life. The purpose of this article is to report the testing and comparison of the measurement structure of the Health Assessment Questionnaire-Disability Index (HAQ-DI), a measure of functional health, in 917 persons living with HIV/AIDS and 901 individuals with rheumatoid arthritis. The samples come from data collected as part of the Arthritis, Rheumatism, and Aging Medical Information System (ARAMIS) and AIDS Time-Oriented Health Outcome Study (ATHOS) projects. Using confirmatory factor analysis (CFA), the hypothesized structure ...