To study trends in the incidence of juvenile rheumatoid arthritis (JRA).
The study covered subjects who were entitled under the nation-wide sickness insurance scheme to receive specially reimbursed medication for juvenile rheumatic diseases in 11 of 21 central hospital districts in Finland (the base population comprised about 445,000 children <16 yr of age) in 1995. Data from the years 1980, 1985 and 1990 were compared with data from 1995 concerning the central part of the area, which had been included in a previous study by us.
A total of 87 incident cases (58 girls and 29 boys) satisfied criteria for JRA in 1995 in the study area. The incidence of JRA was 19.5 per 100 000 [95% confidence interval (CI) 15.6-24.1] of the population <16 yr of age for the whole area. It was 22.7 per 100,000 (95% CI 17.3-29.2) for the area that had been covered by the earlier study (five districts) and 14.9 per 100,000 (95% CI 9.8-21.7) for the new area (six additional districts). The incidence of JRA was significantly higher than in the earlier years (1980, 1985 and 1990) in the same district (trend, P=0.024). The highest incidence, 60.3 per 100,000 (95% CI 35.8-95.4), was noted in 1995 among girls in the age group 10-15 yr in the southernmost part of the study area.
There was both temporal and regional variation in the incidence of JRA. Results of the present study suggest that environmental factors may influence the frequency of JRA.