After years of decline, RA on the rise in women.(NEWS)

SAN FRANCISCO -- An unexpected, significant uptick in rheumatoid arthritis incidence in women may point to increased exposure to hormonal and environmental risk factors, although investigators are unsure of a clear explanation.

"What we had seen over the last 50 years was a continuous decline [in RA incidence] from the 1950s to the [mid] 1990s," Dr. Hilal Maradit Kremers said during a press briefing at the annual meeting of the American College of Rheumatology. "We were expecting that decline to continue."

Surprisingly, the trend instead reversed quite dramatically between 1995 and 2004, according to what began as a rather routine update of incidence trends in Olmsted County, Minn. by Dr. Maradit Kremers and her associates at the Mayo Clinic, Rochester, Minn.

The age-adjusted incidence in women was 54 per 100,000 in the post-1995 analysis, compared with 36.4 per 100,000 in the 1985-2004 time frame. The average age of onset for women was 56.5 years, about the same as in previous years.

Among men, the incidence of RA remained steady between 1995 and 2004, at 28.6 per 100,000, compared with the disease's incidence in previous decades.

Although the Midwestern population included in long-term Mayo Clinic epidemiologic studies underrepresents non-white populations, it has the advantage of stability, allowing disease trends to be detected accurately. Therefore, Dr. Maradit Kremers and her coauthors could say with considerable confidence that the RA incidence among U.S. women is rising.

What remains a mystery to the investigators is why the incidence rate would change, and so rapidly.