Retrospective surveys for multiple sclerosis (MS) carried out up to 1975 in Italy recorded prevalence rates between 5 and 25 cases per 100,000, according to the concept that the Mediterranean latitudes fall into the medium risk zone for MS. In contrast, most studies carried out in Italy from 1975 onwards have recorded prevalences higher than 30, suggesting that most Italian regions are now of high rather than medium risk for MS. However, the use of the prevalence for the study of temporal trends is questionable. Thus, it remains uncertain whether high prevalences revealed by recent studies reflect case ascertainment differences over time or represent a true change in the occurrence of MS. Several incidence studies carried out in Sardinia and the preliminary results of a multicenter study on MS incidence in Italy indicate that the high prevalences recently found in different Italian regions cannot be ascribed to an ascertainment artifact, but they reflect a true change in the incidence of the disease over time.