Since 1994, disability-related costs for medical care and lost productivity have exceeded an estimated $300 billion annually in the United States. To update previous reports on the prevalence and most common causes of disability among adults, CDC and the U.S. Census Bureau analyzed the most recent data from the Survey of Income and Program Participation (SIPP). This report summarizes the findings of that analysis, which indicated that the prevalence of disability in 2005 (21.8%) remained unchanged from 1999 (22.0%); however, because of the aging of the population, particularly the large group born during 1946-1964 ("baby boomers"), the estimated absolute number of persons reporting a disability increased 7.7%, from 44.1 to 47.5 million. The three most common causes of disability continued to be arthritis or rheumatism (affecting an estimated 8.6 million persons), back or spine problems (7.6 million), and heart trouble (3.0 million). Women (24.4%) had a significantly higher prevalence of disability compared with men (19.1%) at all ages. For both sexes, the prevalence of disability doubled in successive age groups (18-44 years, 11.0%; 45-64 years, 23.9%; and >/=65 years, 51.8%). The number of adults reporting a disability likely will increase, along with the need for appropriate medical and public health services, as more persons enter the highest risk age group (>/=65 years). To accommodate the expected increase in demand for disability-related medical and public health services, expanding the reach of effective strategies and interventions aimed at preventing progression to disability and improving disability management in the population is necessary.