Occurrence of rheumatoid arthritis in a nationwide series of twins.

The nationwide Finnish Twin Cohort was linked with the Sickness Insurance Register on the basis of the unique identification number assigned to each Finnish citizen. The study series consisted of 4137 monozygotic (MZ) and 9162 dizygotic (DZ) same-sexed twin pairs born before 1958 and alive in 1975. Altogether, 261 subjects in the series had the right to receive free medication for rheumatoid arthritis (RA) under the Sickness Insurance Act that covers the entire Finnish population. The pairwise concordance percentage for RA was 12.3 in MZ twins and 3.5 in DZ twins. The age and sex adjusted ratio of observed per expected numbers of concordant pairs (relative risk) was 8.6 for MZ pairs and 3.4 for DZ pairs. These figures are lower than those previously reported on twins but compatible with results from family studies on the genetic component of RA.