A significantly higher proportion of Los Angeles County (State of California, USA) multiple sclerosis patient than of King-Pierce Counties (State of Washington, USA) patients, interviewed as part of a prevalence survey, reported adverse perceived effects to heat exposure indicators and beneficial effects from cold exposure indicators. The differences might be at least partially due to the greater awareness of this suspected relationship in Los Angeles. The authors speculate, however, that the more frequent exposure to heat in the Los Angeles area patients may accelerate the pathologic process resulting in the earlier onset and possibly more rapid course observed in Los Angeles area patients.