This article studies risk factors for physical and social disability among U.S. adults ages 55+ who have arthritis, compared to non-arthritis persons of those ages. The dependent variables refer to difficulties in walking, physical functioning (motions and strength), personal care, and household care. The data set is the Supplement on Aging (SOA) (n = 16,148) that accompanied the 1984 National Health Interview Survey. The SOA data are cross-sectional; relationships of risk factors to disability suggest causation but do not directly demonstrate it. Logistic regressions show that risk factors are similar for arthritis and non-arthritis people, with one important exception. (1) The similarities are: For both groups, odds of disability rise with age, diminish with education, and are higher for non-whites and non-married persons. Disability rises with number of chronic diseases and impairments, and it is elevated for underweight persons (Body Mass Index (BMI) less than 20; further analysis indicates this reflects incomplete control of their severe illness status). Long duration of arthritis and recent medical care for it are associated with disability. (2) The exception is: Severe overweight (BMI greater than or equal to 30) is a disability risk factor for arthritis people, but not for non-arthritis people. Previous research has shown that obesity/overweight is a risk factor for etiology of osteoarthritis; our analysis now shows its continued importance for disability when the disease is present.