Concordance rates for systemic rheumatic diseases among monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs ascertained from the older (twins born before 1958) and younger (twins born during the period 1958-1986) parts of the Finnish Twin Cohort were studied. Nine MZ pairs and 10 DZ pairs with at least one member affected by systemic lupus erythematosus (SLE) were identified. Only one MZ pair was concordant for definite SLE by American Rheumatism Association (ARA) criteria. In addition, two more MZ pairs that fulfilled three ARA criteria may have been concordant. None of the DZ pairs were concordant for SLE. MZ twins also had a higher concordance rate for tests of autoantibodies, but the numbers were small. Other systemic rheumatic diseases were infrequent. The cumulative incidence of systemic rheumatic diseases among MZ twin individuals (1.7/1000) was significantly higher than that among DZ twin individuals (0.7/1000).