Multiple sclerosis sibling pairs: clustered onset and familial predisposition.
 We evaluated 48 relapsing-remitting multiple sclerosis (R/R MS) sibling pairs derived from 44 families for age and date of onset of MS symptoms, clinical course, and family history of MS.
 Age- and sex-matched R/R MS clinic patients provided a statistical comparison group.
 The age of onset tended to cluster within multiplex families.
 The initial symptom of MS occurred within 5 years of age in 30/48 sibling pairs compared with 16/48 controls.
 A positive family history of MS (other than siblings) was present in 43% of the multiplex families compared with 20% among simplex controls.
 In 1st-, 2nd-, and 3rd-degree relatives who had lived into the age at risk, 22/1,134 family members of multiplex sibling pairs had probable or definite MS compared with 10/1,215 control family members.
 Age of onset clustering in siblings concordant for R/R MS and an increased risk of MS in other family members suggest that factors influencing disease onset may be in part inherited in these kindreds.
