Coffee consumption may be a risk factor for rheumatoid arthritis. Researchers in Finland found that daily intake of coffee was directly proportional to prevalence of rheumatoid arthritis. People who drank more than four cups a day had over twice the risk of developing rheumatoid factor-positive rheumatoid arthritis.
They suggest that an unidentified ingredient in coffee that could be removed by filtration might trigger the production of rheumatoid factor (Annals of the Rheumatic Diseases 2000:59:631).