Seropositive rheumatoid arthritis in monozygotic twin sisters carrying HLA-DR4 or DR4 associated with B27

We have followed 3 pairs of monozygotic twin sisters with seropositive rheumatoid arthritis (RA) since 1966, 1971 and 1975. RA developed in the probands at the age of 25, 39 and 21 years and in the cotwins 37, 8 and 19 years later, respectively. Two pairs, Nos 1 and 3, were discordant when first seen. Only Pair 1 had a family history of RA. Onset of RA was not related to marital status or pregnancy. It was improbable that contagious infections of childhood or seasonal infections could be important triggers of RA. Since the concordance rate for RA in monozygotic twins depends on the duration of the observation period, the previously stated concordance rate of 30% may be questioned.