Arthritis is a large and growing public health problem in the United States, resulting in costs of $128 billion annually, and continues to be the most common cause of disability. With the aging of the U.S. population, even assuming that the prevalence of obesity and other risk factors remain unchanged, the prevalence of doctor-diagnosed arthritis and arthritis-attributable activity limitation (AAAL) is expected to increase significantly by 2030. To update previous U.S. estimates of doctor-diagnosed arthritis and AAAL, CDC analyzed National Health Interview Survey (NHIS) data from 2007--2009. This report summarizes the results of that analysis, which found that 22.2% (49.9 million) of adults aged ≥18 years had self-reported doctor-diagnosed arthritis, and 9.4% (21.1 million or 42.4% of those with arthritis) had AAAL. Among persons who are obese, an age-adjusted 33.8% of women and 25.2% of men reported doctor-diagnosed arthritis. Arthritis and AAAL represent a major public health problem in the United States that can be addressed, at least in part, by implementing proven obesity prevention strategies and increasing availability of effective physical activity programs and self-management education courses in local communities.