Incidence of rheumatoid arthritis in Finland during 1980-1990.

OBJECTIVE To obtain information on the incidence of rheumatoid arthritis and on its recent trends in Finland. METHODS The study covered those subjects entitled to receive specially reimbursed medication for rheumatoid arthritis under the nationwide sickness insurance scheme in five out of 21 central hospital districts in Finland (population basis about one million adults) during three years: 1980, 1985, and 1990. RESULTS The annual incidence of rheumatoid arthritis in 1980 and 1985, satisfying the American Rheumatism Association 1987 classification criteria, was 39/100,000 of the population > or = 16 years of age. The combined incidence of rheumatoid factor (RF) positive arthritis and RF negative polyarthritis was 46/100,000. A decline of approximately 40% occurred in the number of RF negative rheumatoid arthritis cases in 1990 compared with the earlier years. The declining trend was statistically significant (P = 0.008). CONCLUSION The decline in incidence of RF negative rheumatoid arthritis in Finland may reflect changes in the environment specifically affecting the risk of RF negative disease.