Multiple sclerosis in a nationwide series of twins.

A nation wide Finnish Twin Cohort was linked with sickness insurance and hospital discharge registers on the basis of the unique identification number assigned to each Finnish citizen. The study series consisted of 4,063 monozygotic (MZ) and 9,001 dizygotic (DZ) same-sexed twin pairs born before 1958 and alive in 1981. Altogether, 22 subjects representing 11 MZ pairs and 10 DZ pairs showed clinically definite multiple sclerosis (MS). In one MZ pair both members had the disease. The frequency of MS among DZ twins corresponded to the figure in the Finnish population, but the prevalence was higher in MZ twins than in DZ twins.