To investigate regional differences in the incidence of rheumatoid arthritis (RA).
Those subjects entitled to receive drug reimbursement for chronic inflammatory joint diseases in 11/21 central hospital districts (population base about 1.8 million adults) in Finland during 1995 were studied. The incidence rates from these central hospital districts were compared.
A total of 1213 subjects were entitled to drug reimbursement for chronic inflammatory joint disease which had started at the age of 16 or over. Of these, 598 subjects satisfied the American Rheumatism Association 1987 criteria for RA. The age adjusted incidence of RA was 31.7/100 000 (95% CI 29.2 to 34.4) and varied significantly (p<0.001) among the central hospital districts, ranging from 16.3 to 44.8/100 000.
There are regional differences in the incidence of RA. The reasons for these are probably environmental rather than genetic.