Objective. Based on a small clinical series and previously published case reports, concordance for systemic lupus erythematosus (SLE) among monozygous (MZ) twins has been reported to be as high as 69%. Using a larger and less biased sample, we provide another estimate of this percentage.Methods. We established a registry of twins with SLE, based upon self-reports and information provided by the patients' physicians. We used DNA fingerprinting to validate the reported zygosity in a sample of these twins.Results. firmed by DNA fingerprinting in a subsample of 15 self-described MZ twins and 7 self-described DZ twins. All individuals had correctly predicted their zygosity.Conclusion. MZ concordance for SLE is similar to that for other autoimmune diseases and is much lower than previously believed.Concordance for disease in pairs of twins provides evidence of the relative etiologic contributions of genetic and environmental factors, as well as the timing of these effects. Low rates of concordance for disease over a lifetime, especially among monozygotic (MZ) twins, suggests a greater influence of environment; a large difference in concordance between MZ and dizygotic (DZ) twins suggests a greater influence of genetic determinants. However, for most diseases, there are no population-based twin concordancy data.Although the estimated annual incidence rate for systemic lupus erythematosus (SLE) is 5CL75 per million (1,2), no population-based studies of concordance rates in twins have been published. One case series reported that 69% of the MZ twin pairs were concordant (3), and anecdotal reports describe a similar proportion of concordance among MZ twin pairs. These reports also suggest that a much lower proportion of DZ twin pairs are concordant. In the study presented here, we used a larger sample size and an ascertainment mechanism that is less overtly biased in an effort to derive more meaningful data concerning the etiology of SLE.

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