The British Isles survey of multiple sclerosis in twins.

During a 27-month recruitment period, we identified 146 individuals with multiple sclerosis (MS) who have a twin. A single clinician interviewed and examined 105 pairs of twins, and we confirmed zygosity using minisatellite probes. Including two suspected cases, 11 of 44 (25%) monozygotic twin pairs were concordant compared with two of 61 (3%) dizygotic twin pairs--two of 33 (6%) like-sexed and zero of 28 (0%) opposite-sexed. MRI was performed in 64 of 105 co-twins, and showed abnormalities consistent with demyelination in 13% of monozygotic and 9% of dizygotic co-twins who were clinically unaffected. These findings are similar to the results of most previous studies of MS in twins in which zygosity was not unequivocally established and where the majority of clinically unaffected co-twins were not studied by MRI; the difference in concordance rates in monozygotic and dizygotic twins indicates a significant genetic component in the etiology of MS.