Arthritis is highly prevalent among U.S. adults, the leading cause of disability, and associated with substantial activity limitation, work disability, reduced quality of life, and high health-care costs. As the population ages, arthritis is expected to affect an estimated 67 million adults in the United States by 2030. This report updates estimates of the national prevalence of doctor-diagnosed arthritis and arthritis-attributable activity limitation in the adult U.S. population, using data from the National Health Interview Survey (NHIS) for 2003-2005. The findings indicated that an estimated 21.6% of the adult U.S. population (46.4 million persons) had doctor-diagnosed arthritis, and 8.3% (17.4 million) had arthritis-attributable activity limitations. Public and private health agencies should promote measures to increase the availability of evidence-based arthritis prevention and management interventions.