To update the projected prevalence of self-reported, doctor-diagnosed arthritis and arthritis-attributable activity limitations among US adults ages 18 years and older from 2005 through 2030. Baseline age- and sex-specific prevalence rates of arthritis and activity limitation, using the latest surveillance case definitions, were estimated from the 2003 National Health Interview Survey, which is an annual, cross-sectional, population-based health interview survey of approximately 31,000 adults. These estimates were used to calculate projected arthritis prevalence and activity limitations for 2005-2030 using future population projections obtained from the US Census Bureau. The prevalence of self-reported, doctor-diagnosed arthritis is projected to increase from 47.8 million in 2005 to nearly 67 million by 2030 (25% of the adult population). By 2030, 25 million (9.3% of the adult population) are projected to report arthritis-attributable activity limitations. In 2030, >50% of arthritis cases will be among adults older than age 65 years. However, working-age adults (45-64 years) will account for almost one-third of cases. By 2030, the number of US adults with arthritis and its associated activity limitation is expected to increase substantially, resulting in a large impact on individuals, the health care system, and society in general. The growing epidemic of obesity may also significantly contribute to the future burden of arthritis. Improving access and availability of current clinical and public health interventions aimed at improving quality of life among persons with arthritis through lifestyle changes and disease self-management may help lessen the long-term impact.