Epidemiology experts based the finding on an analysis of population estimates from the U.S. Census Bureau, responses from national surveys and findings from community-based studies across the country. They found two-thirds of arthritis patients are younger than 65 and more than 60 percent are women.

The report, published in Arthritis & Rheumatism, also found that nearly 27 million Americans suffer from osteoarthritis -- the most common type of arthritis. Rising with age, osteoarthritis prevalence also affects the hands and knees of women more frequently than of men, and of African Americans more frequently than of whites.

Rheumatoid arthritis, an inflammatory disease, affects 1.3 million adults. Trends show the average age of diagnosis has increased steadily over time, suggesting that rheumatoid arthritis is becoming a disease of older adults.

In 2005, an estimated 3 million Americans had gout in the previous 12 months -- an inflammatory arthritis linked to elevated uric acid in the blood. Gout tends to be most prevalent among older men.