October 27 (HealthDay News) -- The number of American women with rheumatoid arthritis (RA) is growing again after four decades of decline, according to a Mayo Clinic study.

The researchers tracked 350 adult patients, averaging 56.5 years of age, from Olmsted County, Minn. Of those patients, 69 percent were women.

In the United States, the incidence of RA had steadily declined from 1955 to 1994. But that changed in the mid-1990s, the study found. The Mayo researchers' analysis of data from early 1995 to the start of 2005 revealed that both the incidence and prevalence of RA were rising.

During those 10 years, the incidence of RA among women increased to 54 per 100,000, compared to 36 per 100,000 in the previous 10 years. The incidence of RA among men remained at about 29 per 100,000.

The rate of RA in the overall population increased from 0.85 percent to 0.95 percent.

The reason for the increase isn't clear, but environmental factors may play a role in the rise of the joint disease among women, the researchers suggested.

The study was to be presented at the annual meeting of the American College of Rheumatology/Association of Rheumatology Health Professionals, in San Francisco.

SOURCE: Mayo Clinic, news release, Oct. 25, 2008