Fifty-one twin pairs, one or both members of each with multiple sclerosis (MS), were analyzed for extraneous events occurring prior to onset. Six patient groupings allowed comparison of events with and without the genetic factor. Comparison of monozygotic, discordant (one involved), and dizygotic discordant twins showed a difference. The affected members of the monozygotic pairs encountered prior to onset more birth anoxia, unusual infantile and childhood infections, major operations, and childbirth than did their unaffected twins, a difference not found when comparing dizygotic twins. The difference was most evident in the monozygotic discordant twins interviewed 30 years after onset of MS. If concordant (both involved) twins are analyzed by early vs late age of onset, these events occur at an older age in the late- and at younger age in the early-onset patients, suggesting that they may determine the age of onset of symptoms; they are suspected to be important in the causation of MS in genetically susceptible individuals, although the mechanism of their action is unknown.